brcm: Link CM4's WiFi firmware with DMI machine name.

brcm: Link CM4's WiFi firmware with DMI machine name.

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

The 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.

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