mmc: usdhi6rol0: do not announce UHS capabilities

The driver in its current form does not support UHS at all due to
a missing start_signal_voltage_switch callback.

Also when this callback is added we should let the device tree control
UHS capabilities using the standard mmc bindings.

Signed-off-by: Lars Persson <larper@axis.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>