imx: add macro to detect whether USB PHY is active

imx: add macro to detect whether USB PHY is active

This macro allows to detect whether the USB PHY is active. This
is helpful to detect if the boot ROM has previously started the
USB serial downloader.

The idea is taken from the mfgtool support in the NXP U-Boot:
http://git.freescale.com/git/cgit.cgi/imx/uboot-imx.git/commit/?h=imx_v2016.03_4.1.15_2.0.0_ga&id=a352ed3c5184b95c4c9f7468f5fbb5f43de5e412

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>