pinctrl: mediatek: convert most definitions to const

There exists a situation of the mediatek pinctrl driver that may return
wrong pin function value for the pinmux driver:
- All pin function arrays are defined without const
- Some pin function arrays contain all-zero value, e.g.:
  static int mt7622_spi_funcs[] = { 0, 0, 0, 0, 0, 0, };
- These arrays will be put into .bss section during compilation
- .bss section has no "a" attribute and does not exist in the final binary
  file after objcopy.
- FDT binary blob is appended to the u-boot binary, which occupies the
  .bss section.
- During board_f stage, .bss has not been initialized, and contains the
  data of FDT, which is not full-zero data.
- pinctrl driver is initialized in board_f stage, and it will get wrong
  data if another driver is going to set default pinctrl.

Since pinmux information and soc data are only meant to be read-only, thus
should be declared as const. This will force all pinctrl data being put
into .rodata section. Since .rodata has "a" attribute, even the all-zero
data will be allocated and filled with correct value in to u-boot binary.

diff --git a/drivers/pinctrl/mediatek/pinctrl-mtk-common.h b/drivers/pinctrl/mediatek/pinctrl-mtk-common.h
index 0d9596f..c948b80 100644
--- a/drivers/pinctrl/mediatek/pinctrl-mtk-common.h
+++ b/drivers/pinctrl/mediatek/pinctrl-mtk-common.h
@@ -174,9 +174,9 @@
  */
 struct mtk_group_desc {
 	const char *name;
-	int *pins;
+	const int *pins;
 	int num_pins;
-	void *data;
+	const void *data;
 };
 
 /**
@@ -233,7 +233,7 @@
  */
 struct mtk_pinctrl_priv {
 	void __iomem *base[MAX_BASE_CALC];
-	struct mtk_pinctrl_soc *soc;
+	const struct mtk_pinctrl_soc *soc;
 };
 
 extern const struct pinctrl_ops mtk_pinctrl_ops;
@@ -242,7 +242,7 @@
 void mtk_rmw(struct udevice *dev, u32 reg, u32 mask, u32 set);
 void mtk_i_rmw(struct udevice *dev, u8 i, u32 reg, u32 mask, u32 set);
 int mtk_pinctrl_common_probe(struct udevice *dev,
-			     struct mtk_pinctrl_soc *soc);
+			     const struct mtk_pinctrl_soc *soc);
 
 #if CONFIG_IS_ENABLED(PINCONF)