ppc4xx: Canyonlands: Disable the RTC M41T62 square wave output

This patch disables the square wave output of the M41T62 RTC used on
Canyonlands & Glacier. Here the explanation:

The serial real-time clock part used in the design is an
STMicro M41T62. This part has a full-time 32KHz square wave
output that is connected to the TmrClk input to the
processor. The default state for this square wave output is
enabled so the output runs continuously when the board is
powered normally and also from the battery. The TmrClk input
to the processor goes to ground when the power is removed
from the board/processor, and therefore the running square
wave output is driving ground which drains the battery quickly.

Signed-off-by: Stefan Roese <sr@denx.de>