pinctrl: exynos: Convert to use livetree API for fdt access

Use counterpart dev_read_* functions instead of fdt* ones. It fixes
checkpatch warnings like this:

    WARNING: Use the livetree API (dev_read_...)
    #54: FILE: drivers/pinctrl/exynos/pinctrl-exynos.c:137:
        pinvals[idx] = fdtdec_get_int(fdt, node,

and also makes it possible to avoid using the global data pointer in the
driver.

No functional change.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>