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