ARM: OMAP2+: Move omap_dsp_reserve_sdram_memblock() to mach-omap2

This hardware exists only on 2430 and later omaps, so there's no
need to have it in plat-omap/devices.c.

Note that we don't have any users for exported omap_dsp_get_mempool_base(),
so we can make it static.

Signed-off-by: Tony Lindgren <tony@atomide.com>