am625x_evm_a53: Tweak boot command to set fdt

With the current config for tha SK-AM62, fdtfile isn't set in the U-Boot
environment. When using bootflow to boot from a block device, where the
extlinux.conf file specifies `fdtdir`, a fallback device tree is being
constructed from the `soc` (`k3`) and `board` (`am62x`) environment
variables, resulting in u-Boot trying to retrieve
`/dtbs/6.8.1+/k3-am62x.dtb`. This file doesn't exist.

The environment variables `default_device_tree` and
`default_device_tree_arch` are set in the config, the `findfdt` script
can be called to construct `fdtfile` from the environment variables set
by these config options, however this script currently isn't being run.

Calling this script results in the correct device tree being retrieved:

   Retrieving file: /dtbs/6.8.1+/ti/k3-am625-sk.dtb

Many boards are calling this script as part of their boot command. The
am62x currently isn't. Rectify this so that booting works correctly.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>
Reviewed-by: Christopher Obbard <chris.obbard@collabora.com>