soc: imx8mq: Read SOC revision from TF-A

SOC revision on older imx8mq is not available in fuses so on anything
other than B1 current code just reports "unknown".

TF-A already handles this by parsing the ROM and exposes the value
through a SMC call. Call this instead of reimplementing the workaround
in the kernel itself.

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Reviewed-by: Abel Vesa <abel.vesa@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>