Revert "console: simplify puts()"

This reverts commit c61d0009.

A tbs2910 board user reported a very slow console frambuffer as
regression in current u-boot. I could bisect this down to the
above mentioned commit.

This revert brings back the fast framebuffer console (one
cache flush per string in puts(), not after each char).

Reported-by: Uwe Scheffler <scheffler.u@web.de>
Signed-off-by: Soeren Moch <smoch@web.de>
Tested-by: Uwe Scheffler <scheffler.u@web.de>
Reviewed-by: Simon Glass <sjg@chromium.org>