linux-phytec-ti

fbdev: ssd1307fb: Make reset gpio devicetree property optional

Make reset gpio devicetree property optional. Depending on the board
designing there may not be a dedicated gpio for resetting the
display. Without a proper reset there may be some junk in the display
memory at probe time, so in such a case the display memory is cleared
before turning it on. The devicetree binding document is also updated.

Signed-off-by: Jyri Sarha <jsarha@ti.com>