MAINTAINERS: add entry for staging/fbtft/

Add MAINTAINERS entry for staging/fbtft/
FBTFT is a framework for writing framebuffer drivers
for displays with LCD controllers having onchip RAM.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>