hw/misc/bcm2835_cprman: add a PLL skeleton implementation

hw/misc/bcm2835_cprman: add a PLL skeleton implementation

There are 5 PLLs in the CPRMAN, namely PLL A, C, D, H and B. All of them
take the xosc clock as input and produce a new clock.

This commit adds a skeleton implementation for the PLLs as sub-devices
of the CPRMAN. The PLLs are instantiated and connected internally to the
main oscillator.

Each PLL has 6 registers : CM, A2W_CTRL, A2W_ANA[0,1,2,3], A2W_FRAC. A
write to any of them triggers a call to the (not yet implemented)
pll_update function.

If the main oscillator changes frequency, an update is also triggered.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Luc Michel <luc@lmichel.fr>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>