Although the GPC provides the similar functions for all the
i.MX8M SoC family, the HW register offset and bit defines
still have some slight difference, so move the hw reg
offset & most of the bitfield defines in 'gpc_reg.h' that
is specific to each SoC.

Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Change-Id: I291c435fe98c2f6e6ff8fe0c715ff3a83daa6a0f

Add imx8mn basic support

Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Change-Id: Ibdfcc87700bfaf980e429f3a5fa08515218ae78d