ARM: dts: imx6q-bx50v3: fix at25 spi-clk frequency issue

Change the maxium spi clock frequency from 20MHz to 10MHz to meet the
operation voltage range requirement recommended in AT25 datasheet.

Signed-off-by: Ken Lin <yungching0725@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>