uboot-imx

i.MX28: Move SPL to arch/arm/cpu/arm926ejs/mx28

This moves SPL to common location so it can be reused by multiple boards. Also,
this commit adjusts M28 SoM to avoid breakage due to the move.

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Detlev Zundel <dzu@denx.de>
Cc: Stefano Babic <sbabic@denx.de>