mfd: Convert db8500-prcmu panic() into pr_crit()

panic() is too heavy for this, indeed the PRCMU is critical for
the system but not to the point that we should stop everything,
if we can still get a prompt or so.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>