lib/alloc_phys: Remove locking

With powerpc moving the page allocator, there are no architectures left
which use the physical allocator after the boot setup:  arm, arm64,
s390x and powerpc drain the physical allocator to initialize the page
allocator; and x86 calls setup_vm() to drain the allocator for each of
the tests that allocate memory.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>