arm64: enable FIX_EARLYCON_MEM kconfig

In order to support earlycon on arm64, we need to enable earlycon fixmap
support.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>