omap5912-osk: Fix device initialisation

In the current u-boot, the device pin multiplexing and clock
initialisation needs to be early during the boot process and before
board_init() is called. U-boot is currently crashing on this board
because this is not being done early enough. Therefore, add a s_init()
function for the omap5912-osk board to do this.

Also fix the stack pointer so that it is pointing to the end of the
internal RAM and not the beginning as this was also causing the device
to crash.

Signed-off-by: Jon Hunter <jon-hunter@ti.com>