i.MX28: Improve passing of data from SPL to U-Boot

i.MX28: Improve passing of data from SPL to U-Boot

Pass memory size from SPL via structure located in SRAM instead of SCRATCH
registers. This allows passing more data about boot from SPL to U-Boot, like the
boot mode pads configuration.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Detlev Zundel <dzu@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Wolfgang Denk <wd@denx.de>