omap-common: Add NAND SPL linux booting

This implements booting of Linux from NAND in SPL

Related config parameters:
CONFIG_SYS_NAND_SPL_KERNEL_OFFS
	Offset in NAND of direct boot kernel image to use in SPL
CONFIG_SYS_SPL_ARGS_ADDR
	Address where the kernel boot arguments are expected - this is
	normally RAM-start + 0x100 (on ARM)

Signed-off-by: Simon Schwarz <simonschwarzcor@gmail.com>
CC: Tom Rini <tom.rini@gmail.com>
CC: Stefano Babic <sbabic@denx.de>
CC: Wolfgang Denk <wd@denx.de>