powerpc: macros for e500mc timer regs added

For e500mc cores the watchdog timer period has to be set by means of a
6bit value, that defines the bit of the timebase counter used to signal
a watchdog timer exception on its 0 to 1 transition.
The macro used to set the watchdog period TCR_WP, was redefined for e500mc
to support 6 WP setting.

The parameter (x) given to the macro specifies the prescaling factor of
the time base clock (fTB):

watchdog_period = 1/fTB * 2^x

Signed-off-by: Rainer Boschung <rainer.boschung@keymile.com>
Reviewed-by: York Sun <yorksun@freescale.com>