lib/alloc_phys: Use phys_alloc_aligned_safe and rename it to memalign_early

phys_alloc_aligned_safe() is called only by early_memalign() and the safe
parameter is always true. In the spirit of simplifying the code, merge the
two functions together. Rename it to memalign_early(), to match the naming
scheme used by the page allocator.

Change the type of top_safe to phys_addr_t, to match the type of the top
and base variables describing the available physical memory; this is a
cosmetic change only, since libcflat.h defines phys_addr_t as an alias
for u64.

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