sunxi: Downclock AHB1 to 100MHz on Allwinner A64

sunxi: Downclock AHB1 to 100MHz on Allwinner A64

Currently the AHB1 clock speed is configured as 200MHz by
the SPL, but this causes a subtle and hard to reproduce data
corruption in SRAM C (for example, this can't be easily
detected with a trivial memset/memcmp test).

For what it's worth, the Allwinner's BSP configures AHB1
as 200MHz, as can be verified by running the devmem2 tool
in the system running the Allwinner's kernel 3.10.x:

   0x1C20028: PLL_PERIPH0_CTRL_REG = 0x90041811
   0x1C20054: AHB1_APB1_CFG_REG    = 0x3180
   0x1C20058: APB2_CFG_REG         = 0x1000000
   0x1C2005C: AHB2_CFG_REG         = 0x1

However the FEL mode uses more conservative settings (100MHz
for AHB1):

   0x1C20028: PLL_PERIPH0_CTRL_REG = 0x90041811
   0x1C20054: AHB1_APB1_CFG_REG    = 0x3190
   0x1C20058: APB2_CFG_REG         = 0x1000000
   0x1C2005C: AHB2_CFG_REG         = 0x0

It is yet to be confirmed whether faster AHB1/AHB2 clock settings
can be used safely if we initialize the AXP803 PMIC instead of
using reset defaults. But in order to resolve the data corruption
problem right now, it's best to downclock AHB1 to a safe level.

Note that this issue only affects the SPL, which is not fully
supported on Allwinner A64 yet and it should not affect the boot0
usage (unless somebody can confirm SRAM C corruption with the
boot0 too).

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>