ARM: OMAP2+: am335x: Change the wdt1 func clk src to per_32k clk

WDT1 module can take one of the below clocks as input functional
clock -
     - On-Chip 32K RC Osc [default/reset]
     - 32K from PRCM

The On-Chip 32K RC Osc clock is not an accurate clock-source as per
the design/spec, so as a result, for example, timer which supposed
to get expired @60Sec, but will expire somewhere ~@40Sec, which is
not expected by any use-case.

The solution here is to switch the input clock-source to PRCM
generated 32K clock-source during boot-time itself.

Signed-off-by: Vaibhav Hiremath <hvaibhav@ti.com>
Cc: Santosh Shilimkar <santosh.shilimkar@ti.com>
Cc: Benoit Cousson <benoit.cousson@linaro.org>
Cc: Paul Walmsley <paul@pwsan.com>