arm/arm64: mmu: Rename mmu_get_pte() -> follow_pte()

The function get_pte() from mmu.c returns a pointer to the PTE
associated with the requested virtual address, mapping the virtual
address in the process if it's not already mapped.

mmu_get_pte() returns a pointer to the PTE if and only if the virtual is
mapped in pgtable, otherwise returns NULL. Rename it to follow_pte() to
avoid any confusion with get_pte(). follow_pte() also matches the name
of Linux kernel function with a similar purpose.

Also remove the mmu_enabled() check from the function, as the purpose of
the function is to get the mapping for the virtual address in the pgtable
supplied as the argument, not to translate the virtual address to a
physical address using the current translation; that's what
virt_to_phys() does.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Andrew Jones <andrew.jones@linux.dev>