hw/arm_sysctl: Implement SYS_CFG_OSC function

hw/arm_sysctl: Implement SYS_CFG_OSC function

Implement the SYS_CFG_OSC function. Since the idea of
programmable clock rates doesn't make much sense for QEMU,
we simply allow the clock rate to be read back as written.
The number and value of the daughterboard oscillators varies
between daughterboards, so we provide an array property to
allow their reset values to be configured.

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