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>

arch/arm/include/asm/arch-mx6/sys_proto.h

@@ -6,3 +6,10 @@
  */
 
 #include <asm/mach-imx/sys_proto.h>
+
+#define USBPHY_PWD		0x00000000
+
+#define USBPHY_PWD_RXPWDRX	(1 << 20) /* receiver block power down */
+
+#define is_usbotg_phy_active(void) (!(readl(USB_PHY0_BASE_ADDR + USBPHY_PWD) & \
+				   USBPHY_PWD_RXPWDRX))