arm64: dts: armada-3720-turris-mox: rename u-boot mtd partition to a53-firmware

The partition called "u-boot" in reality contains TF-A and U-Boot, and
TF-A is before U-Boot.

Rename this parition to "a53-firmware" to avoid confusion for users,
since they cannot simply build U-Boot from U-Boot repository and flash
the resulting image there. Instead they have to build the firmware with
the sources from the mox-boot-builder repository [1] and flash the
a53-firmware.bin binary there.

[1] https://gitlab.nic.cz/turris/mox-boot-builder



Signed-off-by: Marek Behún <kabel@kernel.org>
Fixes: 7109d817 ("arm64: dts: marvell: add DTS for Turris Mox")
Cc: Gregory CLEMENT <gregory.clement@bootlin.com>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>