riscv: Add xmipsexectl instructions

Add xmipsexectl instruction opcodes. This includes the MIPS.PAUSE,
MIPS.EHB, and MIPS.IHB instructions.

Signed-off-by: Aleksa Paunovic <aleksa.paunovic@htecgroup.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20250724-p8700-pause-v5-3-a6cbbe1c3412@htecgroup.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>

Author: aleksapaunovic-htec

arch/riscv/include/asm/vendor_extensions/mips.h

@@ -15,4 +15,23 @@ struct riscv_isa_vendor_ext_data_list;
 extern struct riscv_isa_vendor_ext_data_list riscv_isa_vendor_ext_list_mips;
 #endif
 
+/* Extension specific instructions */
+
+/*
+ * All of the xmipsexectl extension instructions are
+ * ‘hint’ encodings of the SLLI instruction,
+ * with rd = 0, rs1 = 0 and imm = 1 for IHB, imm = 3 for EHB,
+ * and imm = 5 for PAUSE.
+ * MIPS.PAUSE is an alternative opcode which is implemented to have the
+ * same behavior as PAUSE on some MIPS RISCV cores.
+ * MIPS.EHB clears all execution hazards before allowing
+ * any subsequent instructions to execute.
+ * MIPS.IHB clears all instruction hazards before
+ * allowing any subsequent instructions to fetch.
+ */
+
+#define MIPS_PAUSE	".4byte 0x00501013\n\t"
+#define MIPS_EHB	".4byte 0x00301013\n\t"
+#define MIPS_IHB	".4byte 0x00101013\n\t"
+
 #endif // _ASM_RISCV_VENDOR_EXTENSIONS_MIPS_H