This patch adds SPL code for the M28 board.

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