firmware: arm_scpi: make it depend on MAILBOX instead of ARM_MHU

ARM_SCPI_PROTOCOL can be used with any mailbox and not just ARM MHU
mailbox controller. This patch drops it's dependency on ARM_MHU and
make it depend on just mailbox framework.

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
diff --git a/drivers/firmware/Kconfig b/drivers/firmware/Kconfig
index 6664f11..41abdc5 100644
--- a/drivers/firmware/Kconfig
+++ b/drivers/firmware/Kconfig
@@ -10,7 +10,7 @@
 
 config ARM_SCPI_PROTOCOL
 	tristate "ARM System Control and Power Interface (SCPI) Message Protocol"
-	depends on ARM_MHU
+	depends on MAILBOX
 	help
 	  System Control and Power Interface (SCPI) Message Protocol is
 	  defined for the purpose of communication between the Application