imx6: Update CONFIG_SPL_STACK defaults in Kconfig

Update the Kconfig entry to have the correct defaults for i.MX6
platforms, and move the existing large comment from imx6_spl.h to
doc/imx/common/imx6.txt so that it's not lost.

Signed-off-by: Tom Rini <trini@konsulko.com>
57 files changed