spi: bcm2835: move to the transfer_one driver model

This also allows for GPIO-CS to get used removing the limitation of
2/3 SPI devises on the SPI bus.

Fixes: spi-cs-high with native CS with multiple devices on the spi-bus
resetting the chip selects to "normal" polarity after a finished
transfer.

No other functionality/improvements added.

Tested with the following 4 devices on the spi-bus:
* mcp2515 with native CS
* mcp2515 with gpio CS
* fb_st7735r with native CS
    (plus spi-cs-high via transistor inverting polarity)
* enc28j60 with gpio-CS
Tested-by: Martin Sperl <kernel@martin.sperl.org>

Signed-off-by: Martin Sperl <kernel@martin.sperl.org>
Signed-off-by: Mark Brown <broonie@kernel.org>