sh: Provide a generic SRAM pool for tiny memories.

This sets up a generic SRAM pool for CPUs and platform code to insert
their otherwise unused memories into. A simple alloc/free interface is
provided (lifed from avr32) for generic code.

This only applies to tiny SRAMs that are otherwise unmanaged, and does
not take in to account the more complex SRAMs sitting behind transfer
engines, or that employ an I/D split.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>