LinuxKPI: add bcd.h

Add bcd2bin() as linuxkpi_bcd2bin(). Libkern does provide a bcd2bin()
which cannot be used leaving us with a conflict (see comment in file).
Fortunately this is only seen in one driver so far and it seems easier
to drop this in and change a single line in the driver than to add this
inline in the driver.

MFC after: 3 days
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D32647