ARM: mach-shmobile: use standard 2MiB coherent DMA memory size

The 158MiB memory area was used to support HD
resolution multimedia workloads using the same
legacy memory allocating solution as on SH.

There are no in-tree kernel dependencies on the
158MiB setting, and future development should
reserve and allocate memory using some other
method like for instance CMA.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Nicolas Pitre <nico@linaro.org>