soc/tegra: pmc: Configure core power request polarity

This patch configures polarity of the core power request signal
in PMC control register based on the device tree property.

PMC asserts and de-asserts power request signal based on it polarity
when it need to power-up and power-down the core rail during SC7.

Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Sowjanya Komatineni <skomatineni@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>