arm_dma_limit stores physical address of maximal address accessible by DMA,
so the phys_addr_t type makes much more sense for it instead of u32. This
patch fixes the following build warning:

arch/arm/mm/init.c:380: warning: comparison of distinct pointer types lacks a cast
Reported-by: NRussell King <linux@arm.linux.org.uk>
Signed-off-by: NMarek Szyprowski <m.szyprowski@samsung.com>