Add missing opamp external outputs for STM32G4

[G-Levine]

This PR adds the ability to use following opamp/ADC combinations when the opamp is configured in external output mode.
These are documented in the STM32G4 reference manual but currently missing in Embassy.

• OPAMP3 with ADC1 (output pin is PB1)
• OPAMP4 with ADC1 (output pin is PB12)
• OPAMP6 with ADC2 (output pin is PB11)

[Dirbaio]

bender run

[G-Levine]

Any idea what caused this CI failure?

+ git clone -n https://github.com/G-Levine/embassy.git code Cloning into 'code'... error: RPC failed; curl 92 HTTP/2 stream 5 was not closed cleanly: CANCEL (err 8) error: 4178 bytes of body are still expected fetch-pack: unexpected disconnect while reading sideband packet fatal: early EOF fatal: fetch-pack: invalid index-pack output run failed: exited with code 128

[Dirbaio]

flaky networking. retrying

bender run