On the MX31litekit, the bootloader seems to communicate with the MC13783
PMIC chip before booting Linux. However, it does not flush all the
buffers properly after that, which makes the imx-spi driver read
bogus data when probing the MC13783.

Fix that by draining the SPI receive buffer on startup.
Signed-off-by: Daniel Mack <daniel@caiaq.de>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>