mmc_fdt: handle broken-cd property

The documented properties [1] for card-detection are one of:

• cd-gpios
• non-removable
• broken-cd

In cd_setup() we handle the first two, but not the latter, resulting in
a silently undetected card on an affected system.

To work around this, force cd_disabled when broken-cd is specified, so
that the card detect helper function gets to run. A more complete
solution would implement some kind of polling mechanism to detect the
card's presence or removal.

Some variants of the Allwinner D1, such as the Lichee Rv, specify this
property in the mmc0 device node.

[1] sys/contrib/device-tree/Bindings/mmc/mmc-controller.yaml

Reported by: Haowu Ge <gehaowu@bitmoe.com>
Tested by: Haowu Ge <gehaowu@bitmoe.com>
Reviewed by: imp, manu, mmel
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D53546

(cherry picked from commit 768ee6d454821cc63247cb4ffe526c5a06accff0)