ARM: i.MX7: describe USB serial download boot mode

Due to i.MX7 erratum e10574: "Watchdog: A watchdog timeout or software
trigger will not reset the SOC", we can't do warm reset via imxwd-warm
for BootROM reboot mode as we do on other i.MX SoCs.

What we can do instead though, is use the SoC's reset controller to
toggle the Cortex-A7's reset. This will have us reenter BootROM with
GPR registers intact. Forcing serial download on the i.MX7 now possible
with:

  gpr.reboot_mode.next=serial reset -w

Note that the new restart handler is not fit for general purpose.
Depending on boot medium, it may hang, because the BootROM may not
reinitialize the peripheral properly.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Link: https://lore.barebox.org/20221017071000.1458292-8-a.fatoum@pengutronix.de
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

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