arm64: dts: marvell: armada-37xx: move firmware node to generic dtsi file

Move the turris-mox-rwtm firmware node from Turris MOX' device tree into
the generic armada-37xx.dtsi file and use the generic compatible string
'marvell,armada-3700-rwtm-firmware' instead of the current one.

Turris MOX DTS file contains also old compatible string for backward
compatibility.

The Turris MOX rWTM firmware can be used on any Armada 37xx device,
giving them access to the rWTM hardware random number generator, which
is otherwise unavailable.

This change allows Linux to load the turris-mox-rwtm.ko module on these
boards.

Tested on ESPRESSObin v5 with both default Marvell WTMI firmware and
CZ.NIC's firmware. With default WTMI firmware the turris-mox-rwtm fails
to probe, while with CZ.NIC's firmware it registers the HW random number
generator.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <kabel@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>