arm64: Move device scmi to std.arm

The scmi driver in its current form requires the arm_doorbell
driver to communicate with the firmware.
The arm_doorbell is only found in ARM Juno reference board (and
apparently on Morello too).
If we want to use scmi on other platform (like some rockchip or imx
soc), the driver needs to be updated to support svc/shmem communication
with the firmware.
For now since it can be only used with arm_doorbell move the device to
std.arm otherwise kernel configs like ALLWINNER or ROCKCHIP fails to build.

Reviewed by:	br, imp
Sponsored by:	Beckhoff Automation GmbH & Co. KG
Differential Revision:	https://reviews.freebsd.org/D37953

sys/arm64/conf/std.arm

@@ -11,6 +11,9 @@ device		pl061			# Arm PL061 GPIO controller
 # Serial (COM) ports
 device		pl011
 
+# Firmware
+device		scmi			# System Control Management Interface
+
 device		arm_doorbell		# ARM Message Handling Unit (MHU)
 
 options 	FDT

sys/arm64/conf/std.dev

@@ -109,6 +109,4 @@ options 	HID_DEBUG		# enable debug msgs
 device		hid			# Generic HID support
 
 # Firmware
-device		scmi			# System Control Management Interface
-
 device		mmio_sram		# Generic on-chip SRAM