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>

arch/arm/mach-imx/spl.c

@@ -31,6 +31,18 @@ u32 spl_boot_device(void)
 	if (((bmode >> 24) & 0x03) == 0x01) /* Serial Downloader */
 		return BOOT_DEVICE_BOARD;
 
+	/*
+	 * The above method does not detect that the boot ROM used
+	 * serial downloader in case the boot ROM decided to use the
+	 * serial downloader as a fall back (primary boot source failed).
+	 *
+	 * Infer that the boot ROM used the USB serial downloader by
+	 * checking whether the USB PHY is currently active... This
+	 * assumes that SPL did not (yet) initialize the USB PHY...
+	 */
+	if (is_usbotg_phy_active())
+		return BOOT_DEVICE_BOARD;
+
 	/* BOOT_CFG1[7:4] - see IMX6DQRM Table 8-8 */
 	switch ((reg & IMX6_BMODE_MASK) >> IMX6_BMODE_SHIFT) {
 	 /* EIM: See 8.5.1, Table 8-9 */