mailbox: Improve RISCV_SBI_MPXY_MBOX guidance

RISC-V SBI Message Proxy (MPXY) Mailbox support defaults to enabled, but
the help text states "If unsure say N".

Recommend enabling this driver, as it is a very critical RISC-V driver
providing mailbox channels to other drivers such as clock, system MSIs,
etc.

Fixes: bf3022a ("mailbox: Add RISC-V SBI message proxy (MPXY) based mailbox driver")
Suggested-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>

Author: geertu

drivers/mailbox/Kconfig

@@ -396,7 +396,7 @@ config RISCV_SBI_MPXY_MBOX
 	  Mailbox driver implementation for RISC-V SBI Message Proxy (MPXY)
 	  extension. This mailbox driver is used to send messages to the
 	  remote processor through the SBI implementation (M-mode firmware
-	  or HS-mode hypervisor). Say Y here if you want to have this support.
-	  If unsure say N.
+	  or HS-mode hypervisor). Say Y here, unless you are sure you do not
+	  need this.
 
 endif