ARM: Allow arm_memblock_steal() to remove memory from any RAM region

Allow arm_memblock_steal() to remove memory from any RAM region,
including highmem areas.  This allows memory to be stolen from the
very top of declared memory, including highmem areas, rather than
our precious lowmem.

Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>