arm64: Split out platform options to separate Kconfig

Let's move out the platform Kconfig entries to a separate file, since these
changes usually get moved through arm-soc instead of the arm64 arch tree, and
this will lead to fewer conflicts

Signed-off-by: Olof Johansson <olof@lixom.net>
Acked-by: Will Deacon <will.deacon@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>