ARM: dts: am57xx-beagle-x15: Update Phy supplies

Originally, all the SoC PHY rails were supplied by LDO3. However, as a
result of characterization, it was determined that this posed a risk in
extreme load  conditions. Hence the PHY rails are split between two
different LDOs. Update the related node as a result

LDO3/VDDA_1V8_PHYA supplies vdda_usb1, vdda_usb2, vdda_sata, vdda_usb3
LDO4/VDDA_1V8_PHYB supplies vdda_pcie1, vdda_pcie0, vdda_hdmi, vdda_pcie

NOTE: We break compatibility with pre-production boards with this change
since, the PMIC LDO4 is disabled at OTP level.

The new configuration is the plan of record and all pre-production
boards are supposed to be replaced with the latest boards matching the
mentioned configuration.

Some very few 10 something boards have been created and
stopped production till the latest modifications were done (PMIC USB
interrupt, LDO4 etc) - and all of those boards are now getting
scrapped.. If there are any (as per tracking information, there should
not be any), TI should be contacted to have them replaced.

Signed-off-by: Nishanth Menon <nm@ti.com>
[tony@atomide.com: updated commit about these being TI internal protos]
Signed-off-by: Tony Lindgren <tony@atomide.com>