ARM: tegra: Fix PWM clock programming

PWM clock source registers in Tegra 2 have different clock source selection bit
fields than other registers.  PWM clock source bits in CLK_SOURCE_PWM_0 register
are located at bit field bit[30:28] while others are at bit field bit[31:30] in
their respective clock source register.

This patch updates the clock programming to correctly reflect that, by adding a
flag to indicate the alternate bit field format and checking for it when
selecting a clock source (parent clock).

Signed-off-by: Bill Huang <bilhuang@nvidia.com>
Signed-off-by: Simon Que <sque@chromium.org>
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Stephen Warren <swarren@nvidia.com>