mpc52xx/wdt: re-enable the MPC5200 WDT

This set of patches merges the MPC5200 WDT into the GPT code, making it functional again - currently, the MPC5200 GPT code blocks using the WDT.

Additionally, it defines a new OF property as to reserve and/or enable the WDT during the boot process which may be a requirement for safety-related (e.g. ISO/EN 61508) applications.

Note that all patches are against Grant Likely's "next" tree.

Cheers, Albrecht.