pinctrl: mediatek: convert most definitions to const

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.

Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>