doc: board: phytec: k3: Document boot flow and watchdog

Add two short sections to the common K3 phyCORE docs.
Describe the default boot flow and its deprecated version.
And write down the use of the watchdog.

Signed-off-by: Wadim Egorov <w.egorov@phytec.de>

doc/board/phytec/k3-common.rst

@@ -1,6 +1,53 @@
 .. SPDX-License-Identifier: GPL-2.0+
 .. sectionauthor:: Wadim Egorov <w.egorov@phytec.de>
 
+Boot Flow
+---------
+
+The default `bootcmd` performs three steps:
+
+.. code-block::
+
+  run start_watchdog; bootflow scan -lb; run ${boot}boot
+
+Boot devices are scanned in the order given by `boot_targets`:
+
+.. code-block::
+
+  mmc1 mmc0 spi_flash dhcp
+
+For each device, U-Boot tries the boot methods listed in `bootmeths`:
+
+.. code-block::
+
+  [rauc] script efi extlinux pxe
+
+The `rauc` bootmeth is only present when `CONFIG_BOOTMETH_RAUC=y` is set in
+the A53 defconfig. RAUC slot selection is handled entirely by the bootmeth;
+no environment-side configuration is required.
+
+The legacy `${boot}boot` chain (`mmcboot`, `spiboot`, `netboot`) is kept for
+backwards compatibility and prints a deprecation warning when run. New
+deployments should rely on the standard boot mechanism (`bootflow`) only.
+
+
+Watchdog
+--------
+
+`bootcmd` runs `start_watchdog` before starting the boot flow. When
+`CONFIG_WATCHDOG_TIMEOUT_MSECS` is set to a non-zero value and the
+`watchdog` environment variable points to a watchdog device, U-Boot enables
+the watchdog with that timeout.
+
+After this point the OS is responsible for servicing the watchdog. If it
+does not feed the watchdog before the timeout expires, the SoC will reset.
+Make sure the watchdog driver is enabled and configured in the kernel and
+userspace before relying on this.
+
+To skip the watchdog start, either build with `CONFIG_WATCHDOG_TIMEOUT_MSECS=0`
+or set `watchdog_timeout_ms=0` in the environment.
+
+
 Environment
 -----------