lib/alloc_phys: Initialize align_min

Commit 11c4715f ("alloc: implement free") changed align_min from a
static variable to a field for the alloc_ops struct and carried over the
initializer value of DEFAULT_MINIMUM_ALIGNMENT.

Commit 7e3e823b ("lib/alloc.h: remove align_min from struct
alloc_ops") removed the align_min field and changed it back to a static
variable, but missed initializing it.

Initialize align_min to DEFAULT_MINIMUM_ALIGNMENT, as it was intended.

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