watchdog: mpc8xxx: Make it generic

mpc8xx, mpc83xx and mpc86xx have similar watchdog with almost same
memory registers.

Refactor the driver to get the register addresses from the
device tree and use the compatible to know the prescale factor.

Calculate the watchdog setup value from the provided timeout.

Don't declare it anymore as an HW_WATCHDOG, u-boot will start
servicing the watchdog early enough.

On mpc8xx the watchdog configuration register is also used for
configuring the bus monitor. So add it as an option to the watchdog
when it is mpc8xx. When watchdog is not selected, leave the
configuration of the initial SYPCR from Kconfig.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>