arm: imx: mx7: Move CONFIG_OPTEE_TZDRAM_SIZE from lib/optee

This config is only used by three boards with this SOC. Most other
platforms derive this information from devicetree, and are unlikely
to ever need this config.

Moreover, it is confusing when Kconfig asks for this value under
"Support OPTEE images", but does not do anything with the value.
Move it to imx7 for those boards who still make use of it.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>