video: st7586s: Fix the st7568s code to handle the new type of display

- use 3 pixels per byte instead of 2

Signed-off-by: Björn Krombholz <b.krombholz@pironex.de>
arm: pia-am335x-em: Fix exported display buttons and leds

Signed-off-by: Björn Krombholz <b.krombholz@pironex.de>