This implements the missing callback hooks nrf52_spi0/1/2/3register
that are usually used with mmcsd for card detection.
This also stubs out the missing spi trigger function which is not
used on this platform.
The card detect was tested with the nRF52-feather board and a
modified KeyBoard FeatherWing.
Signed-off-by: Brennan Ashton <bashton@brennanashton.com>
1473c6848f