mmc: bcmstb: Add support for bcm2712 SD controller

Make sure that core SDHCI accessors are used and add
device specific card detection initialization, which
is borrowed from vendor Linux driver code.

Tested-by: Jens Maus <mail@jens-maus.de>
Tested-by: Darko Alavanja <darko.alavanja@konsulko.com>
Signed-off-by: Ivan T. Ivanov <iivanov@suse.de>
Signed-off-by: Matthias Brugger <mbrugger@suse.com>