arm64: add basic Kconfig symbols for i.MX8

Add basic Kconfig symbols to make the MXC architecture available
in the ARM64 world.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Abel Vesa <abel.vesa@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>