MFC: r241679

MFC: r241679

It turns out that as documented, PCF8563_R_SECOND_VL (i.e. battery low)
doesn't automatically clear when VDD rises above Vlow again and needs to be
cleared manually. However, apparently this needs all of the time registers
to be set, i.e. pcf8563_settime(), and not just PCF8563_R_SECOND in order
for PCF8563_R_SECOND_VL to stick. Thus, we just issue a warning during
pcf8563_attach() rather than failing with ENXIO in case it is set.

Approved by:	re (kib)

git-svn-id: svn://svn.freebsd.org/base/releng/9.1@241813 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f