hw/arm/spitz: Implement inbound GPIO lines for bit5 and power signals

hw/arm/spitz: Implement inbound GPIO lines for bit5 and power signals

Currently the Spitz board uses a nasty hack for the GPIO lines
that pass "bit5" and "power" information to the LCD controller:
the lcdtg realize function sets a global variable to point to
the instance it just realized, and then the functions spitz_bl_power()
and spitz_bl_bit5() use that to find the device they are changing
the internal state of. There is a comment reading:
 FIXME: Implement GPIO properly and remove this hack.
which was added in 2009.

Implement GPIO properly and remove this hack.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20200628142429.17111-6-peter.maydell@linaro.org