Set it to max. allowed 375kHz for faster transfers. The limit is given
by the erratum [1].

[1] https://www.nxp.com/docs/en/errata/IMX6DQCE.pdfSigned-off-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>