ARM: dts: imx6ull-tarragon: Reduce SPI clock for QCA7000

ARM: dts: imx6ull-tarragon: Reduce SPI clock for QCA7000

Our hardware department recently informed us that, according to
the specification, the QCA7000 should be operated with a
maximum SPI clock frequency of 12 MHz. Even if it appears to work
at a higher frequency, we should not take any risks here. A short
performance test showed no measurable loss of speed.

Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>