ARM: dts: imxrt1050: increase mmc max-frequency property

According to i.MXRT1050 Datasheet usdhc supports up to 200Mhz clock so
let's increase max-frequency property to 200Mhz.

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Acked-by: Jesse Taube <mr.bossman075@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>