brcm: Link RPi4's WiFi firmware with DMI machine name.

brcm: Link RPi4's WiFi firmware with DMI machine name.

The kernel when booted in ACPI mode with SMBIOS/DMI
data uses that data to set the machine manufacture/model.

The RPi's WiFi driver uses the machine string to
select the correct firmware image. Because of
this, an additional link to select the correct
firmware is needed.

Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Peter Robinson <pbrobinson@gmail.com>
Signed-off-by: Josh Boyer <jwboyer@kernel.org>