mfd: pm8921: Disable driver until it gets fixed

The pm8921 driver has been broken for a while now, but was prevented
from compiling because the SSBI bus driver was missing.  Now that SSBI
is present, pm8921 causes compile fails.

Until the pm8921 driver is fixed, mark it as BROKEN to prevent
compiles from failing.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: David Brown <davidb@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>