arm64: mm: ignore memory above supported physical address size

When booting a kernel without 52-bit PA support (e.g. a kernel with 4k
pages) on a system with 52-bit memory, the kernel will currently try to
use the 52-bit memory and crash. Fix this by ignoring any memory higher
than what the kernel supports.

Fixes: f77d2817 ("arm64: enable 52-bit physical address support")
Signed-off-by: Kristina Martsenko <kristina.martsenko@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>