video: st7586s: add fb driver for ST7586S spi display

merged single commit based on:
- fea27068e636eb73176ed751031401ae5a2ce23d
- 5c9b568b7349a4823192eef3aca5e607c9fbc1c8
- 9bb40b9312ec12e91a05855f2d9433262fe58844
- a854a983679d9282251438dac813b4af99c7a156
- 15851a9f176170b2437b714225ebfd77b6a75270
- 700a24c00407c667ec1cb60244961ab0047f128a
from: https://github.com/Satius/pia-linux-kernel.git

Changes to original commit:
- added generic copyright header (drive was already marked as GPL)
- remove references to specific board files

Signed-off-by: Björn Krombholz <b.krombholz@pironex.de>
video: st7586s: fix compiler warnings

Signed-off-by: Björn Krombholz <b.krombholz@pironex.de>
video: st7586s: update formatting to match kernel standards

Signed-off-by: Björn Krombholz <b.krombholz@pironex.de>
arm: pia-am335x-em: enable support for spi display

Signed-off-by: Björn Krombholz <b.krombholz@pironex.de>
arm: pia-am335x-em: add support for RS485 DE GPIOs

Signed-off-by: Björn Krombholz <b.krombholz@pironex.de>
arm: pia-am335x-em: don't init DISP LED as LED but generic GPIO

- also make sure the display is turned on

Signed-off-by: Björn Krombholz <b.krombholz@pironex.de>
arm: pia-am335x-em: don't turn the driver of the debug rs485 off

Signed-off-by: Björn Krombholz <b.krombholz@pironex.de>
iio: max536: allow ref voltage from platform data

Signed-off-by: Björn Krombholz <b.krombholz@pironex.de>
arm: pia-am335x-em: add pinmux for WLAN gpios

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