pinctrl: renesas: sh-pfc: Implement .pin_config_group_get() callback

pinctrl: renesas: sh-pfc: Implement .pin_config_group_get() callback

When reading /sys/kernel/debug/pinctrl/*.pinctrl-sh-pfc/pinconf-groups
while CONFIG_DEBUG_PINCTRL is enabled, the user is confronted with a
seemlingly endless stream of identical messages on the console:

    sh-pfc e6060000.pinctrl: cannot get configuration for pin group, missing group config get function in driver

Fix this by implementing the sh_pfc_pinconf_ops.pin_config_group_get()
callback.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://patch.msgid.link/130ce567f23fd6eef8f5fa7273480a0e3ff2d1d9.1777562482.git.geert+renesas@glider.be