ARM: dts: am57xx-beagle-x15: Add support for rev B1

ARM: dts: am57xx-beagle-x15: Add support for rev B1

Latest update to the BeagleBoard-X15 platform (revision B1)[1] updates
for allowing UHS SD cards to function with the split of supply to SD
card from a dedicated LDO.

As a result of this, AM57xx BeagleBoard-X15 now uses gpio2_30 instead
of gpio6_28 for HDMI because HDMI_LS_OE should now be switched from
GPIO6_28(Y9) to GPIO2_30 (AG8) to avoid a 1.8V GPIO toggling a 3.3V
SoC input when the SD card is in UHS 1.8V mode.

NOTE: For UHS mode to function, we need full fledged IODelay support
in kernel to be functional. IODelay support is yet to be added.

Further, It does not make much sense to spin off a new board
compatible flag since there is no real functional benefit for the
same.

Note: Even though production version is supposed to be B1, there is
over ~200 boards of previous version (A2)[2] out there which continue
to get supported with the existing dts file (to maintain compatibility
with existing bootloaders for A2) and the production board is now
supported as revb1.

[1] https://github.com/beagleboard/beagleboard-x15/blob/master/BEAGLEBOARD_X15_REV_B1.pdf
[2] http://marc.info/?l=linux-arm-kernel&m=147273929820708&w=2

Tested-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>