clk: imx: pllv4: add fractional-N pll support

The pllv4 supports fractional-N function, the formula is:

PLL output freq = input * (mult + num/denom),

This patch adds fractional-N function support, including
clock round rate, calculate rate and set rate, with this
patch, the clock rate of APLL in clock tree is more accurate
than before:

Without fraction:
apll_pre_sel                      1        1        1    24000000          0     0  50000
   apll_pre_div                   1        1        2    24000000          0     0  50000
      apll                        1        1        2   528000000          0     0  50000
         apll_pfd3                0        0        0   792000000          0     0  50000
         apll_pfd2                0        0        0   339428571          0     0  50000
         apll_pfd1                0        0        0   352000000          0     0  50000
            usdhc0                0        0        0   352000000          0     0  50000
         apll_pfd0                1        1        1   352000000          0     0  50000

With fraction:
apll_pre_sel                      1        1        1    24000000          0     0  50000
   apll_pre_div                   1        1        2    24000000          0     0  50000
      apll                        1        1        2   529200000          0     0  50000
         apll_pfd3                0        0        0   793800000          0     0  50000
         apll_pfd2                0        0        0   340200000          0     0  50000
         apll_pfd1                0        0        0   352800000          0     0  50000
            usdhc0                0        0        0   352800000          0     0  50000
         apll_pfd0                1        1        1   352800000          0     0  50000

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>