imx_common: detect USB serial downloader reliably

The current mechanism using SCR/GPR registers work well when
the serial downloader boot mode has been selected explicitly
(either via boot mode pins or using bmode command). However,
in case the system entered boot ROM due to unbootable primary
boot devices (e.g. empty eMMC), the SPL fails to detect that
it has been downloaded through serial loader and tries to
continue booting from eMMC:
  Trying to boot from MMC1
  mmc_load_image_raw_sector: mmc block read error
  SPL: failed to boot from all boot devices
  ### ERROR ### Please RESET the board ###

The only known way to reliably detect USB serial downloader
is by checking the USB PHY receiver block power state...

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Acked-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Tested-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Eric Nelson <eric@nelint.com>