spl: stm32f7: configure for xip booting

spl: stm32f7: configure for xip booting

With xip booting configuration, we don't need to copy the next image
(U-Boot or linux xipimage) from flash to sdram area.

Flash memory organization is like this:
	spl-U-Boot:	u-boot-spl.bin 	: 0x0800_0000
	U-Boot :	u-boot-dtb.bin	: 0x0800_8000
	linux :		xipImage	: 0x0800_8000

It is also possible to have U-Boot binary & linux binaries configured at
different addresses of flash memory like U-Boot at 0x0800_8000 & linux
xipImage at 0x0800_4000. But in any case, spl-U-Boot needs to be compiled for
U-Boot as next binary with SPL_OS_BOOT option disabled.
By default, spl is configured to boot linux xipImage.

Signed-off-by: Vikas Manocha <vikas.manocha@st.com>