mailbox: Improve RISCV_SBI_MPXY_MBOX guidance

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: bf3022a4eb119c6b ("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>