ARM: mm: Fix max_mapnr with recent max*pfn updates

With commit  26ba47b1 {ARM: 7805/1: mm: change max*pfn to include
the physical offset of memory}, the max_pfn already contain
PHYS_PFN_OFFSET, so it shouldn't be taken into account again.

While at it, use use set_max_mapnr() helper.

Cc: Russell King <linux@arm.linux.org.uk>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Nicolas Pitre <nicolas.pitre@linaro.org>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>