arm64: ignore DT memreserve entries when booting in UEFI mode

UEFI provides its own method for marking regions to reserve, via the
memory map which is also used to initialise memblock. So when using the
UEFI memory map, ignore any memreserve entries present in the DT.

Reported-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>