soc: imxrt: Clean up INIT_ARM_PLL config

Don't forcefully select this config in SOC level. Make it softer default
y so board can unselect it.

The config should not be possible if there is no arm pll, namely on
RT101x and RT102x. So add dependency clause about this.

And of course, code for this was a mess, clean up a bit.

Also remove the ifdeffry for selecting a default value for the two SOCs,
because they already put the same default value in the SOC Devicetree
DTSI, so that code had no purpose as long as a board didn't completely
redefine the SOC DT.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>