SPL: Add XIP booting support

Enable support for XIP (execute in place) of U-Boot or kernel image. There is
no need to copy image from flash to ram if flash supports execute in place.

Signed-off-by: Vikas Manocha <vikas.manocha@st.com>
Reviewed-by: Alexandru Gagniuc <alex.g@adaptrum.com>

arch/arm/include/asm/spl.h

@@ -29,6 +29,7 @@ enum {
 	BOOT_DEVICE_I2C,
 	BOOT_DEVICE_BOARD,
 	BOOT_DEVICE_DFU,
+	BOOT_DEVICE_XIP,
 	BOOT_DEVICE_NONE
 };
 #endif