ARM: head.S: rename PMD_ORDER to PMD_ENTRY_ORDER

ARM: head.S: rename PMD_ORDER to PMD_ENTRY_ORDER

PMD_ORDER denotes order of magnitude for a PMD entry, i.e PMD entry size
is 2 ^ PMD_ORDER.

Rename PMD_ORDER to PMD_ENTRY_ORDER to allow a generic definition of
PMD_ORDER as order of a PMD allocation: (PMD_SHIFT - PAGE_SHIFT).

Link: https://lkml.kernel.org/r/20220705154708.181258-16-rppt@kernel.org
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Acked-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>