微處理器 - Allwinner H3 (NanoPi M1) - 如何初始化R_GPIO

參考資料：
https://wiki.iagent.no/wiki/AR100
https://wiki.friendlyelec.com/wiki/index.php/NanoPi_M1
https://forum.armbian.com/topic/5051-nanopi-neo2-cpu-frequency-issue/page/2/
https://github.com/friendlyarm/u-boot/tree/sunxi-v2017.x/arch/arm/include/asm/arch-sunxi

似乎每次遇到抄襲別人東西的時候就會缺少說明文件，Allwinner的R_GPIO(R_PIO)就是一個例子，在使用Allwinner H3 R_GPIO之前，PIO_GATING(BUS_CLK_GATING_REG2)必須先開啟，接著初始化PRCM相關暫存器，可惜，這部份並沒有說明文件，因此，只能去看U-Boot如何做的，然後照抄一遍

  .equ PRCM,      0x01f01400
  .equ CCU,       0x01c20000
  .equ PIO,       0x01c20800

  .equ BUS_CLK_GATING_REG2,  (CCU + 0x68)
  .equ PRCM_APB0_GATE,       (PRCM + 0x28)
  .equ PRCM_APB0_RESET,      (PRCM + 0xb0)
  .equ PRCM_SEC_SWITCH,      (PRCM + 0x1d0)

reset:
  ldr r0, =BUS_CLK_GATING_REG2
  ldr r1, [r0]
  orr r1, #(1 << 5)
  str r1, [r0]

  ldr r0, =PRCM_SEC_SWITCH
  ldr r1, =(1 << 2) | (1 << 1) | (1 << 0)
  str r1, [r0]

  ldr r0, =PRCM_APB0_GATE
  ldr r1, [r0]
  orr r1, #(1 << 0)
  str r1, [r0]

  ldr r0, =PRCM_APB0_RESET
  ldr r1, =(1 << 0)
  str r1, [r0]