ARM: at91: fix at91sam9263ek DT mmc pinmuxing settings

ARM: at91: fix at91sam9263ek DT mmc pinmuxing settings

As discovered on a custom board similar to at91sam9263ek and basing
its devicetree on that one apparently the pin muxing doesn't get
set up properly. This was discovered since the custom boards u-boot
does funky stuff with the pin muxing and leaved it set to SPI
which made the MMC driver not work under Linux.
The fix is simply to define the given configuration as the default.
This probably worked by pure luck before, but it's better to
make the muxing explicitly set.

Signed-off-by: Andreas Henriksson <andreas.henriksson@endian.se>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: <stable@vger.kernel.org>  # 3.11+