imx8: cpu: add uclass based CPU driver

imx8: cpu: add uclass based CPU driver

print_cpuinfo() in board init code requires uclass CPU driver,
add it to be able to display CPU info when CONFIG_DISPLAY_CPUINFO
option is enabled. CPU node in DT will have to include 'clocks'
and 'u-boot,dm-pre-reloc' properties for generic print_cpuinfo()
to work as expected. The driver outputs info for i.MX8QXP Rev A
and Rev B CPUs.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>