soc: nxp_imx: fix ENET_PLL selection for MIMXRT1021

Kconfig currently emits a warning when building for a MIMXRT1021 SoC:

  warning: INIT_ENET_PLL (defined at
  soc/arm/nxp_imx/rt/Kconfig.defconfig.mimxrt1052:66) has direct
  dependencies NET_L2_ETHERNET && SOC_MIMXRT1052 && SOC_SERIES_IMX_RT
  with value n, but is currently being y-selected by the following
  symbols:
   - SOC_MIMXRT1021 (defined at soc/arm/nxp_imx/rt/Kconfig.soc:12),
  with value y, direct dependencies <choice> (value: y), and select
  condition <choice> (value: y)

This is due to the fact the ENET PLL is the one used by the ARM core on
the MIMXRT1021 so it is always enabled, while it is declared protected
by NET_L2_ETHERNET and SOC_SERIES_IMX_RT.

Fix that by declaring the symbol in Kconfig.defconfig.series.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>