The GSI layer used for IPA requires firmware to be loaded.

Currently either the AP or the modem loads the firmware,
distinguished by whether the "modem-init" Device Tree
property is defined.

Some newer systems implement a third option.  In preparation for
that, encapsulate the code that determines how the GSI firmware
gets loaded in a new function, ipa_firmware_loader().
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>