spi: a3700: Allow to enable or disable FIFO mode

The armada 3700 SPI controller allows to make transfers without using
the 32 bytes RFIFO and WFIFO.

This commit enable switching between FIFO and non-FIFO mode, which is
necessary to implement full-duplex transfers.

Signed-off-by: Maxime Chevallier <maxime.chevallier@smile.fr>
Signed-off-by: Mark Brown <broonie@kernel.org>