Introduce common timer functions

Many platforms duplicate pretty much the same timer code yet they all have
a 32-bit freerunning counter register. Create a common implementation that
minimally requires 2 or 3 defines to add timer support:

CONFIG_SYS_TIMER_RATE - Clock rate of the timer counter
CONFIG_SYS_TIMER_COUNTER - Address of 32-bit counter
CONFIG_SYS_TIMER_COUNTS_DOWN - Define if counter counts down

All functions are weak or ifdef'ed so they can still be overriden by any
platform.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>