ARM: OMAP2+: PM: introduce the power domains functional states

Here is a re-spin after some comments and suggestions after review and discussions.

Implement the functional states for the power domains:
- unify the API to use the functional states. The new API consists of the pwrdm_set*_fpwrst and pwrdm_read*_fpwrst functions and is the API to use to control the power domains power and logic states,
- reorganize the powerdomain API in internal and external parts, in powerdomain.h [1]
- protect the power domain state change by a lock in the functions that read and set the powerdomains next functional state,
- introduce the functional states for power domains power states and logic power states [2], and the conversion functions between the functional and internal states. The conversion functions are lightweight and generic. The power domains allowed states [3] are defined in the pwrsts and pwrsts_logic_ret fields of the struct

powerdomain,
- program the logic power state of power domains from the functional states, in pwrdm_set*_fpwrst
- convert the OMAP2/3/4 PM code to use the updated API,
- provide the power domains statistics by functional states,
- provide ftrace tracepoints with the functional state,
- provide error logs in critical code, which makes the development easier.