arm64: dts: librem5-devkit: allow the redpine card to be removed

By adding broken-cd to the usdhc2 stanza the Redpine card can be
detected when the HKS is turned off and on.

Signed-off-by: Angus Ainslie (Purism) <angus@akkea.ca>
Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>