hw/arm_sysctl: Implement SYS_CFG_VOLT

hw/arm_sysctl: Implement SYS_CFG_VOLT

Implement the SYS_CFG_VOLT registers which return the voltage
of various supplies on motherboard and daughterboard. Since
QEMU implements a perfectly stable power supply these registers
always return a constant value. The number and value of the
daughterboard voltages is dependent on the specific daughterboard,
so we use a property array to allow the board to configure them
appropriately.

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