hw/arm_sysctl: Implement SYS_CFG_MUXFPGA writes as a no-op

hw/arm_sysctl: Implement SYS_CFG_MUXFPGA writes as a no-op

SYS_CFG_MUXFPGA allows the guest to select whether the
video output should come from the motherboard's LCD
controller or the daughterboard's one. Since QEMU doesn't
currently support selecting the video output like this,
implement as a no-op, so Linux doesn't complain about
the register not being implemented.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>