ARM: OMAP4: Use WARN_ON() instead of BUG_ON() with graceful exit

OMAP4 L2X0 initialisation code uses BUG_ON() for the ioremap()
failure scenarios.

Use WARN_ON() instead and allow graceful function exits.

This was suggsted by Kevin Hilman <khilman@ti.com> during
OMAP4 PM code review.

Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Acked-by: Jean Pihet <j-pihet@ti.com>
Reviewed-by: Kevin Hilman <khilman@ti.com>
Tested-by: Vishwanath BS <vishwanath.bs@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>