regulator: rename dev_pdata to plat

Simplify the subsystem by renaming `dev_pdata` to just `plat`.
No functional change, just trivial renaming.

Suggested-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Eugen Hristev <eugen.hristev@collabora.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>