clk: sunxi-ng: sun50i: h6: Modify GPU clock configuration to support DFS

Using simple bash script it was discovered that not all CCU registers
can be safely used for DFS, e.g.:

    while true
    do
        devmem 0x3001030 4 0xb0003e02
        devmem 0x3001030 4 0xb0001e02
    done

Script above changes the GPU_PLL multiplier register value. While the
script is running, the user should interact with the user interface.

Using this method the following results were obtained:

| Register  | Name           | Bits  | Values | Result |
| --        | --             | --    | --     | --     |
| 0x3001030 | GPU_PLL.MULT   | 15..8 | 20-62  | OK     |
| 0x3001030 | GPU_PLL.INDIV  |     1 | 0-1    | OK     |
| 0x3001030 | GPU_PLL.OUTDIV |     0 | 0-1    | FAIL   |
| 0x3001670 | GPU_CLK.DIV    |  3..0 | ANY    | FAIL   |

DVFS started to work seamlessly once dividers which caused the
glitches were set to fixed values.

Signed-off-by: Roman Stratiienko <[email protected]>
Reviewed-by: Jernej Skrabec <[email protected]>
Signed-off-by: Jernej Skrabec <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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