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. Change it back to being initialized with the value
DEFAULT_MINIMUM_ALIGNMENT.

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