ARM: dts: am57xx-beagle-x15: set VDD_SD to always-on

LDO1 regulator (VDD_SD) is connected to SoC's vddshv8. vddshv8 needs to
be kept always powered (see commit 5a0f93c6 ("ARM: dts: Add
am57xx-beagle-x15"), but at the moment VDD_SD is enabled/disabled
depending on whether an SD card is inserted or not.

This patch sets LDO1 regulator to always-on.

This patch has a side effect of fixing another issue, HDMI DDC not
working when SD card is not inserted:

Why this happens is that the tpd12s015 (HDMI level shifter/ESD
protection chip) has LS_OE GPIO input, which needs to be enabled for the
HDMI DDC to work. LS_OE comes from gpio6_28. The pin that provides
gpio6_28 is powered by vddshv8, and vddshv8 comes from VDD_SD.

So when SD card is not inserted, VDD_SD is disabled, and LS_OE stays
off.

The proper fix for the HDMI DDC issue would be to maybe have the pinctrl
framework manage the pin specific power.

Apparently this fixes also a third issue (copy paste from Kishon's
patch):

ldo1_reg in addition to being connected to the io lines is also
connected to the card detect line. On card removal, omap_hsmmc
driver does a regulator_disable causing card detect line to be
pulled down. This raises a card insertion interrupt and once the
MMC core detects there is no card inserted, it does a
regulator disable which again raises a card insertion interrupt.
This happens in a loop causing infinite MMC interrupts.

Fixes: 5a0f93c6 ("ARM: dts: Add am57xx-beagle-x15")
Cc: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reported-by: Louis McCarthy <compeoree@gmail.com>
Acked-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>