pinctrl: realtek: Tag unused pins as __maybe_unused

pinctrl: realtek: Tag unused pins as __maybe_unused

These pin definitions are helpful to have when working with the
driver in the future, so they are in a sense a bit like
documentation. They could be commented out as well, but why.

This should fix these build warnings:

drivers/pinctrl/realtek/pinctrl-rtd1315e.c:231:35: warning:
'rtd1315e_boot_sel_pins' defined but not used [-Wunused-const-variable=]
drivers/pinctrl/realtek/pinctrl-rtd1315e.c:231:35: warning:
'rtd1315e_reset_n_pins' defined but not used [-Wunused-const-variable=]
drivers/pinctrl/realtek/pinctrl-rtd1315e.c:231:35: warning:
'rtd1315e_scan_switch_pins' defined but not used [-Wunused-const-variable=]
drivers/pinctrl/realtek/pinctrl-rtd1315e.c:231:35: warning:
'rtd1315e_testmode_pins' defined but not used [-Wunused-const-variable=]
drivers/pinctrl/realtek/pinctrl-rtd1315e.c:231:35: warning:
'rtd1315e_wd_rset_pins' defined but not used [-Wunused-const-variable=]
drivers/pinctrl/realtek/pinctrl-rtd1319d.c:237:35: warning:
'rtd1319d_boot_sel_pins' defined but not used [-Wunused-const-variable=]
drivers/pinctrl/realtek/pinctrl-rtd1319d.c:237:35: warning:
'rtd1319d_reset_n_pins' defined but not used [-Wunused-const-variable=]
drivers/pinctrl/realtek/pinctrl-rtd1319d.c:237:35: warning:
'rtd1319d_scan_switch_pins' defined but not used [-Wunused-const-variable=]
drivers/pinctrl/realtek/pinctrl-rtd1319d.c:237:35: warning:
'rtd1319d_testmode_pins' defined but not used [-Wunused-const-variable=]
drivers/pinctrl/realtek/pinctrl-rtd1319d.c:237:35: warning:
'rtd1319d_wd_rset_pins' defined but not used [-Wunused-const-variable=]

Cc: Tzuyi Chang <tychang@realtek.com>
Link: https://lore.kernel.org/oe-kbuild-all/202309270313.mBEc9o1A-lkp@intel.com/
Link: https://lore.kernel.org/oe-kbuild-all/202309270448.7Aen3Sgx-lkp@intel.com/
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20231006-fix-realtek-warnings-v1-1-09af253312ba@linaro.org