soc/tegra: pmc: Select REGMAP

Commit 9d5e7c3e ("soc/tegra: pmc: Provide USB sleepwalk register
map") introduced a dependency on the regmap infrastructure, so select
the corresponding Kconfig entry. This avoids failures in random builds
that may otherwise end up having SOC_TEGRA_PMC enabled but not REGMAP.

Signed-off-by: Thierry Reding <treding@nvidia.com>