BACKPORT: arm64: kaslr: randomize the linear region

When KASLR is enabled (CONFIG_RANDOMIZE_BASE=y), and entropy has been
provided by the bootloader, randomize the placement of RAM inside the
linear region if sufficient space is available. For instance, on a 4KB
granule/3 levels kernel, the linear region is 256 GB in size, and we can
choose any 1 GB aligned offset that is far enough from the top of the
address space to fit the distance between the start of the lowest memblock
and the top of the highest memblock.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Bug: 30369029
Patchset: kaslr-arm64-4.4

(cherry picked from commit 031a4213c11a5db475f528c182f7b3858df11db)
Signed-off-by: Jeff Vander Stoep <jeffv@google.com>
Change-Id: I272de8ee358351d95eacc7dc5f47600adec3e813