mmc: mvsdio: delete platform data code path

There are no in-kernel users of the MVSDIO platform data method
(instantiating from a board file) so just delete this code and
make this a DT-only driver. We depend on OF and check that we have
an OF node in probe().

Cc: Nicolas Pitre <nico@fluxnic.net>
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Nicolas Pitre <nico@linaro.org>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>