spi: rockchip: use irq rather than polling

Register an interrupt handler to fill/empty the
tx and rx fifos rather than busy-looping.

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