boot-wrapper-aarch64: Do not hardcode TEXT_LIMIT

We hard code TEXT_LIMIT to check for image overflow, assuming the memory
is always at 0x80000000, which may not always be true. Instead use the
offset from the PHYS_OFFSET, which is actually dependent on the DT.

Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>