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: NSantosh Shilimkar <santosh.shilimkar@ti.com>
Acked-by: NJean Pihet <j-pihet@ti.com>
Reviewed-by: NKevin Hilman <khilman@ti.com>
Tested-by: NVishwanath BS <vishwanath.bs@ti.com>
Signed-off-by: NKevin Hilman <khilman@ti.com>

arch/arm/mach-omap2/omap4-common.c

@@ -72,7 +72,8 @@ static int __init omap_l2_cache_init(void)
 
 	/* Static mapping, never released */
 	l2cache_base = ioremap(OMAP44XX_L2CACHE_BASE, SZ_4K);
-	BUG_ON(!l2cache_base);
+	if (WARN_ON(!l2cache_base))
+		return -ENOMEM;
 
 	/*
 	 * 16-way associativity, parity disabled