pinctrl: pass -DDEBUG in subdirs

When drivers are compiled in subdirectories the -DDEBUG flag need
to be passed in the individual Makefiles.
Reported-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Suggested-by: Yingjoe Chen <yingjoe.chen@mediatek.com>
Cc: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/pinctrl/Makefile

 # generic pinmux support
 
-ccflags-$(CONFIG_DEBUG_PINCTRL)	+= -DDEBUG
+subdir-ccflags-$(CONFIG_DEBUG_PINCTRL)	+= -DDEBUG
 
 obj-$(CONFIG_PINCTRL)		+= core.o pinctrl-utils.o
 obj-$(CONFIG_PINMUX)		+= pinmux.o