mfd: MAX77693: Fix NULL pointer error when initializing irqs

This patch initialize register map of MUIC device because mfd driver
of Maxim MAX77693 use regmap-muic instance of MUIC device when irqs of
Maxim MAX77693 is initialized before call max77693-muic probe() function.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Myungjoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Reported-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>