net: ipa: introduce "qcom,gsi-loader" property

Introduce a new way of specifying how the GSI firmware gets loaded
for IPA.  Currently, this is indicated by the presence or absence of
the Boolean "modem-init" Device Tree property.  The new property
must have a value--either "self" or "modem"--which indicates whether
the AP or modem is the GSI firmware loader, respectively.

For legacy systems, the new property will not exist, and the
"modem-init" property will be used.  For newer systems, the
"qcom,gsi-loader" property *must* exist, and must have one of the
two prescribed values.  It is an error to have both properties
defined, and it is an error for the new property to have an
unrecognized value.

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