dt-bindings: net: qcom,ipa: add firmware-name property

Add a new optional firmware-name property to the IPA DT node.  It
is used only if the modem is not doing early initialization (i.e.,
if the modem-init property is not present).  Its value is the name
of the firmware file to use; if it's not specified, a default name
("ipa_fws.mdt") is used.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>