mailbox: Add RISC-V SBI message proxy (MPXY) based mailbox driver

Add a mailbox controller driver for the new SBI message proxy extension
which is part of the SBI v3.0 specification.

Acked-by: Jassi Brar <jassisinghbrar@gmail.com>
Co-developed-by: Rahul Pathak <rpathak@ventanamicro.com>
Signed-off-by: Rahul Pathak <rpathak@ventanamicro.com>
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Link: https://lore.kernel.org/r/20250818040920.272664-8-apatel@ventanamicro.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>

drivers/mailbox/Kconfig

@@ -369,4 +369,15 @@ config BCM74110_MAILBOX
 	  processor and coprocessor that handles various power management task
 	  and more.
 
+config RISCV_SBI_MPXY_MBOX
+	tristate "RISC-V SBI Message Proxy (MPXY) Mailbox"
+	depends on RISCV_SBI
+	default RISCV
+	help
+	  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.
+
 endif

drivers/mailbox/Makefile

@@ -78,3 +78,5 @@ obj-$(CONFIG_THEAD_TH1520_MBOX)	+= mailbox-th1520.o
 obj-$(CONFIG_CIX_MBOX)	+= cix-mailbox.o
 
 obj-$(CONFIG_BCM74110_MAILBOX)	+= bcm74110-mailbox.o
+
+obj-$(CONFIG_RISCV_SBI_MPXY_MBOX)	+= riscv-sbi-mpxy-mbox.o