spi: rockchip: support 4bit words

The hardware supports 4, 8 and 16bit spi words,
so add the missing support for 4bit words.

Signed-off-by: Emil Renner Berthing <kernel@esmil.dk>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Mark Brown <broonie@kernel.org>