The NCR, MDM_CTL* and AUD registers manipulate the state of external
signals (eg, the RTS, DTR signals and the ethernet oscillator enable
signal) or indicate the state of external signals (eg, CTS, DSR).

Where these registers can be written, the current value can be read
back, which relieves us from having to maintain a software copy of
the current state.

Model these registers as fixed-direction GPIO registers.
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>