dt-bindings: riscv: Add B ISA extension description

Add description of the single-letter B extension for Bit Manipulation.
B is mandatory for RVA23U64.

The B extension is ratified in the 20240411 version of the unprivileged
ISA specification. According to the ratified spec, the B standard
extension comprises instructions provided by the Zba, Zbb, and Zbs
extensions.

Add two-way dependency check to enforce that B implies Zba/Zbb/Zbs; and
when Zba/Zbb/Zbs (all of them) are specified, then B must be added too.

The reason why B/Zba/Zbb/Zbs must coexist at the same time is that
unlike other single-letter extensions, B was ratified (Apr/2024) much
later than its component extensions Zba/Zbb/Zbs (Jun/2021).

When "b" is specified, zba/zbb/zbs must be present to ensure
backward compatibility with existing software and kernels that only
look for the explicit component strings.

When all three components zba/zbb/zbs are specified, "b" should also be
present. Making "b" mandatory when all three components are present.

Existing devicetrees with zba/zbb/zbs but without "b" will generate
warnings that can be fixed in follow-up patches.

Signed-off-by: Guodong Xu <guodong@riscstar.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>

Author: docularxu

Documentation/devicetree/bindings/riscv/extensions.yaml

@@ -109,6 +109,13 @@ properties:
             The standard C extension for compressed instructions, as ratified in
             the 20191213 version of the unprivileged ISA specification.
 
+        - const: b
+          description:
+            The standard B extension for bit manipulation instructions, as
+            ratified in the 20240411 version of the unprivileged ISA
+            specification. The B standard extension comprises instructions
+            provided by the Zba, Zbb, and Zbs extensions.
+
         - const: v
           description:
             The standard V extension for vector operations, as ratified
@@ -738,6 +745,30 @@ properties:
         then:
           contains:
             const: f
+      # B comprises Zba, Zbb, and Zbs
+      - if:
+          contains:
+            const: b
+        then:
+          allOf:
+            - contains:
+                const: zba
+            - contains:
+                const: zbb
+            - contains:
+                const: zbs
+      # Zba, Zbb, Zbs together require B
+      - if:
+          allOf:
+            - contains:
+                const: zba
+            - contains:
+                const: zbb
+            - contains:
+                const: zbs
+        then:
+          contains:
+            const: b
       # Zcb depends on Zca
       - if:
           contains: