ARM: imx: Make PLL settings configurable at board level

Staticize intpll_configure(). Add weak board_imx_intpll_override()
function which can be defined at board level to override specific
PLL frequency settings early during boot. This can be used to for
example force faster CPU core clock frequency if the hardware can
handle it.

Example of increasing CPU core clock to 1600 MHz on i.MX8M Plus:
```
int board_imx_intpll_override(enum pll_clocks pll, ulong *freq)
{
       if (pll == ANATOP_ARM_PLL)
               *freq = MHZ(1600);
       return 0;
}
```

Signed-off-by: Marek Vasut <marex@denx.de>