arm: mx6: ddr: do not write into reserved bit

Bit 16 in mapsr register is in a reserved field. Don't write to it.

Cc: Stefano Babic <sbabic@denx.de>
Cc: Tim Harvey <tharvey@gateworks.com>
Acked-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>