The old sdtweak overlay allowed the SD interface to be effectively
disabled unless there was a card present at boot time, but that
overlay doesn't work on bcm2711 and has largely been replaced by
a set of sd_* dtparams (which have the advantage of being board-
specific.

Add an sd_poll_once dtparam to allow the same functionality on
all Raspberry Pi boards.

See: https://github.com/raspberrypi/linux/issues/3286

Signed-off-by: Phil Elwell <phil@raspberrypi.org>