soc/tegra: pmc: Correct type of variable for tegra_pmc_readl()

The function tegra_pmc_readl() returns the u32 type data and hence
change the data type of variable where this data is stored to u32
type.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Reviewed-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>