Make sure value in register r0 and r1 is preserved and passed to
the board_init_ll() and mxs_common_spl_init() where it can be
processed further. The value in r0 can be configured during the
BootStream generation to arbitary value, r1 contains pointer to
return value from CALL'd function.

This patch also clears the value in r0 before returning to BootROM
to make sure the BootROM is not confused by this value.

Finally, this patch cleans up some comments in the start.S file.

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