dt-bindings: net: qcom,ipa: support skipping GSI firmware load

Add a new enumerated value to those defined for the qcom,gsi-loader
property.  If the qcom,gsi-loader is "skip", the GSI firmware will
already be loaded, so neither the AP nor modem is required to load
GSI firmware.

Signed-off-by: Alex Elder <elder@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>