sh: select memchunk size using kernel cmdline

Allow user to pass parameters on kernel command line to override
default size for physically contiguous memory buffers. The default
VPU buffer size is too small for VGA harware encoding, but instead
of just bumping up the number we allow the user to override the
default size using the command line. Supports SuperH Mobile hardware
blocks such as VEU, VPU and CEU.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>