fsi: aspeed: Support cabled FSI

Some FSI capable systems have internal FSI signals, and some have
external cabled FSI. Software can detect which machine this is by
reading a jumper GPIO, and also control which pins the signals are
routed to through a mux GPIO.

This attempts to find the GPIOs at probe time. If they are not present
in the device tree the driver will not error and continue as before.

The mux GPIO is owned by the FSI driver to ensure it is not modified at
runtime. The routing jumper obtained as non-exclusive to allow other
software to inspect it's state.

Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20200728025527.174503-3-joel@jms.id.au
Signed-off-by: Joel Stanley <joel@jms.id.au>

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