arm64/efi: efistub: don't abort if base of DRAM is occupied

If we cannot relocate the kernel Image to its preferred offset of base of DRAM
plus TEXT_OFFSET, instead relocate it to the lowest available 2 MB boundary plus
TEXT_OFFSET. We may lose a bit of memory at the low end, but we can still
proceed normally otherwise.

Acked-by: Mark Salter <msalter@redhat.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Leif Lindholm <leif.lindholm@linaro.org>
Tested-by: Leif Lindholm <leif.lindholm@linaro.org>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>