ARM: imx: Support larger SPL size on IMX6DQ

Previously the SPL size on all iMX6 platforms was restricted to 68KB
because the OCRAM size on iMX6SL/DL parts is only 128KB. However, the
other iMX6 variants have 256KB of OCRAM. Add an option
CONFIG_MX6_OCRAM_256KB which allows using the full size on boards which
don't need to support the SL/DL variants. This allows for an SPL size of
196KB, which makes it much easier to use configurations such as SPL with
driver model and FDT control.

Signed-off-by: Robert Hancock <hancock@sedsystems.ca>
Tested-by: Adam Ford <aford173@gmail.com> #imx6q_logic