mips: cmd: go: Flush cache before jumping to app/image

It has been noticed on MT7628/88 platforms, that booting the RAM image
does not work reliably. Sometimes it works and sometimes not. Debugging
showed that this "might" be a cache related issue as very strange
errors occurred (e.g. output corrupted etc).

This patch adds a cache flush for the complete SDRAM area to the go cmd
before jumping to the entry point for the MIPS architecture. The
complete area is flushed as we don't know at this point, how big the
area of the "application" really is.

Signed-off-by: Stefan Roese <sr@denx.de>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Tested-by: Mauro Condarelli <mc5686@mclink.it>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: Mauro Condarelli <mc5686@mclink.it>
Cc: Weijie Gao <weijie.gao@mediatek.com>