powerpc: Consolidate bootcount_{store|load} for PowerPC

This patch consolidates bootcount_{store|load} for PowerPC by
implementing a common version in arch/powerpc/lib/bootcount.c. This
code is now used by all PowerPC variants that currently have these
functions implemented.

The functions now use the proper IO-accessor functions to read/write the
values.

This code also supports two different bootcount versions:

a) Use 2 separate words (2 * 32bit) to store the bootcounter
b) Use only 1 word (2 * 16bit) to store the bootcounter

Version b) was already used by MPC5xxx.

Signed-off-by: Stefan Roese <sr@denx.de>
Acked-by: Detlev Zundel <dzu@denx.de>
Acked-by: Kim Phillips <kim.phillips@freescale.com>
          for 83xx parts
Cc: Michael Zaidman <michael.zaidman@gmail.com>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Kim Phillips <kim.phillips@freescale.com>
Cc: Anatolij Gustschin <agust@denx.de>