examples/sdmmc: enable internal pull-ups in SD mode

examples/sdmmc: enable internal pull-ups in SD mode

Some development boards do not have sufficient external pull-ups on
SD card pins. Most notably, if high level on GPIO13 is not registered
by the card when GO_IDLE_STATE command is received, the card will enter
SPI mode and further communication will not be possible, until power to
the card is toggled.

This change enables internal pull-ups on SD card pins in an attempt to
make this initialization process more reliable.