mm/ARM: fix ARMs __ffs() to conform to avoid warning with NO_BOOTMEM

mm/ARM: fix ARMs __ffs() to conform to avoid warning with NO_BOOTMEM

Building ARM with NO_BOOTMEM generates below warning.

mm/nobootmem.c: In function _____free_pages_memory___:
mm/nobootmem.c:88:11: warning: comparison of distinct pointer types lacks a cast

	order = min(MAX_ORDER - 1UL, __ffs(start));

ARM's __ffs() differs from other architectures in that it ends up being
an int, whereas almost everyone else is unsigned long.

So fix ARMs __ffs() to conform to other architectures.  Suggested by
Russell King.

Some more details in below thread -
	https://lkml.org/lkml/2013/12/9/807

Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>