davinci: add SRAM allocator

Provide a generic SRAM allocator using genalloc, and vaguely
modeled after what AVR32 uses.  This builds on top of the
static CPU mapping set up in the previous patch, and returns
DMA mappings as requested (if possible).

Compared to its OMAP cousin, there's no current support for
(currently non-existent) DaVinci power management code running
in SRAM; and this has ways to deallocate, instead of being
allocate-only.

The initial user of this should probably be the audio code,
because EDMA from DDR is subject to various dropouts on at
least DM355 and DM6446 chips.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>