regulator: rtmv20: Add Richtek to Kconfig text

The other Richtek drivers has Richtek prefix, make it consistent.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Link: https://lore.kernel.org/r/20210530124101.477727-2-axel.lin@ingics.com
Signed-off-by: Mark Brown <broonie@kernel.org>