ARM: OMAP: Allocate McBSP devices dynamically

Based on Chandra's earlier patches in linux-omap tree.

Note that omap1_mcbsp_check and omap2_mcbsp_check are no longer
needed as there's now omap_mcbsp_check_valid_id() defined.

Also some functions can now be marked __init.

Signed-off-by: Chandra Shekhar <x0044955@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>