kernel: 5.10: allocate last level PTEs in high memory

kernel: 5.10: allocate last level PTEs in high memory

Enable support for allocating user space page table entries in high memory [1],
for the targets which support this feature. This saves precious low memory
(permanently mapped, the only type of memory directly accessible by the kernel).

[1] https://www.kernel.org/doc/html/latest/vm/highmem.html

Signed-off-by: Rui Salvaterra <rsalvaterra@gmail.com>