soc: imx: increase build coverage for imx8m soc driver

The soc-imx8.c driver is actually for i.MX8M family, so rename it
to soc-imx8m.c.

Use CONFIG_SOC_IMX8M as build gate, not CONFIG_ARCH_MXC, to control
whether build this driver, also make it possible for compile test.

Default set it to y for ARCH_MXC && ARM64

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>