arm64: dts: meson: fix mmc v2 chips max frequencies

According the datasheets, emmc v2 chips (gxbb and gxl) don't support
more that 100Mhz in UHS-1 SD modes and HS in SDIO.

Align the max-frequency to 100MHz for UHS-1 and 50MHz for HS

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>