omap4: control: Add accessor api's for pad control module

On OMAP4 control pad are not addressable from control
core base. So the common omap_ctrl_read/write APIs breaks
Hence export separate APIs to manage the omap4 pad control
registers.

This APIs will work only for OMAP4

Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>