Input: snvs_pwrkey - make it depend on ARCH_MXC

The SNVS power key is not only used on i.MX6SX and i.MX7D, it is also
used by i.MX6UL and NXP's latest ARMv8 based i.MX8M series SOC. So
update the config dependency to use ARCH_MXC, and add the COMPILE_TEST
too.

Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>