net: ipa: permit GSI firmware loading to be skipped

Define a new value "skip" for the "qcom,gsi-loader" Device Tree
property.  If used, it indicates that neither the AP nor the modem
need to load GSI firmware (because it has already been loaded--for
example by the boot loader).

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>