pinctrl: renesas: r8a7779: Optimize fixed-width reserved fields

Describe registers with fixed-width register fields and many reserved
fields using the PINMUX_CFG_REG_VAR() macro, as the latter supports a
shorthand not requiring dummy values.

This reduces kernel size by 81 bytes.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/ecc7377d2992694226dcf055bed0b617701a3d71.1649865241.git.geert+renesas@glider.be

drivers/pinctrl/renesas/pfc-r8a7779.c

@@ -3300,13 +3300,10 @@ static const struct pinmux_cfg_reg pinmux_config_regs[] = {
 		GP_5_1_FN, FN_A2,
 		GP_5_0_FN, FN_A1 ))
 	},
-	{ PINMUX_CFG_REG("GPSR6", 0xfffc001c, 32, 1, GROUP(
-		0, 0, 0, 0, 0, 0, 0, 0,	0, 0, 0, 0, 0, 0, 0, 0,
-		0, 0, 0, 0, 0, 0, 0, 0,	0, 0, 0, 0, 0, 0, 0, 0,
-		0, 0, 0, 0, 0, 0, 0, 0,
-		0, 0,
-		0, 0,
-		0, 0,
+	{ PINMUX_CFG_REG_VAR("GPSR6", 0xfffc001c, 32,
+			     GROUP(-23, 1, 1, 1, 1, 1, 1, 1, 1, 1),
+			     GROUP(
+		/* GP6_31_9 RESERVED */
 		GP_6_8_FN, FN_IP3_20,
 		GP_6_7_FN, FN_IP3_19,
 		GP_6_6_FN, FN_IP3_18,