ion: don't use __arm_ioremap to map pages

ion_heap_map_kernel already implements mapping a scatterlist of
pages into the kernel, and all heaps are required to have struct
pages associated with them, so delete the functions that use
__arm_ioremap and use ion_heap_map_kernel instead.

Change-Id: Ia2dfd8d8c6e719d7d2f68dd4c458826fdb938260
Signed-off-by: Colin Cross <ccross@android.com>