ARM: dts: imx6ul-evk: Fix peripheral regulator

Many peripherals are affected by gpio5/2, not just sensors. One of those
is ethernet phy so network boot is current broken.

Fix by renaming reg_sensors and marking it as "always on". Also add a
comment asking for careful testing if this is to be made dynamic in the
future.

The "peri_3v3" naming is similar to imx6sx-sdb and regulator-name is
same string as in schematics (VPERI_3V3).

Fixes: 09e2b104

 ("ARM: dts: imx6ul-14x14-evk: Add sensors' GPIO regulator")
Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Reviewed-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>