staging: sm750fb: add sm750 to staging

sm750 of Silicon Motion is pci-e display controller device and has
features like dual display and 2D acceleration. This patch adds the
driver to staging.

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>