dt-bindings: net: qcom,ipa: restate a requirement

Either the AP or modem loads GSI firmware.  If the modem-init
property is present, the modem loads it.  Otherwise, the AP loads
it, and in that case the memory-region property must be defined.

Currently this requirement is expressed as one or the other of the
modem-init or the memory-region property being required.  But it's
harmless for the memory-region to be present if the modem is loading
firmware (it'll just be ignored).

Restate the requirement so that the memory-region property is
required only if modem-init is not present.

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