stm32: stm32f7: add spl build support

This commit supports booting from stm32 internal nor flash. spl U-Boot
initializes the sdram memory, copies next image (e.g. standard U-Boot)
to sdram & then jumps to entry point.

Here are the flash memory addresses for U-Boot-spl & standard U-Boot:
	- spl U-Boot		: 0x0800_0000
	- standard U-Boot	: 0x0800_8000

To compile u-boot without spl: Remove SUPPORT_SPL configuration
(arch/arm/mach-stm32/Kconfig)

Signed-off-by: Vikas Manocha <vikas.manocha@st.com>
[trini: Rework Kconfig logic a bit]
Signed-off-by: Tom Rini <trini@konsulko.com>