soc: nxp: rt: kconfig: Clean up INIT_ENET_PLL handling

Move the definition of INIT_ENET_PLL to soc/arm/nxp_imx/rt/Kconfig.soc,
which is where the other INIT_*_PLL symbols are defined, and consistenly
enable it with 'select' like for the other symbols, instead of via
Kconfig.defconfig.mimxrt{1052,1062,1064}.

Remove an old empty INIT_ENET_PLL definition from
soc/arm/nxp_imx/rt/Kconfig.defconfig.series, which was just there to
hack around a dependency on NET_L2_ETHERNET. If the symbol does not
depend on NET_L2_ETHERNET in all cases, it's better to not add the
dependency.

Also add a help text to hint what's going on there.

Found with a script (INIT_ENET_PLL was only defined in Kconfig.defconfig
files).

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>