i.MX28: Add function to adjust memory parameters

i.MX28: Add function to adjust memory parameters

This function can be overridden at run-time and allows implementors
of new boards based on the i.MX28 chip to fine-tune the memory params.
It is possible to write into the dram_vals array because when the SPL
runs, it is located SRAM. Therefore the location is writable. There is
no possibility of these data to be read-only.

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