ARM: imx6: fix min/max voltage of anatop 2p5 regulator

The regulation bound of this regulator are 2.1V to 2.875V, the
wrong DT values cause the driver to miscalculate the effective
voltage.

This isn't really an issue right now, as nobody actively changes
the regulator voltage, but better fix it now.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>