ARM: 7993/1: mm/memblock: add memblock_get_current_limit

Apart from setting the limit of memblock, it's also useful to be able
to get the limit to avoid recalculating it every time. Add the function
to do so.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Acked-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>