pinctrl: samsung: add pin_dbg_show callback for debugfs

This patch adds a samsung_pin_dbg_show function to implement the
pin_dbg_show callback function which can be used to show pin
confuration values. Basically, it can show pin setting values by
accessing the "pins" node like below:

$ cat pins
pin 0 (gpy7-0)  CON(0x0) DAT(0x1) PUD(0x1) DRV(0x0) CON_PDN(0x0) PUD_PDN(0x0)

Signed-off-by: Chanho Park <parkch98@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Tested-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>