ARM: dts: meson8b: ec100: improve the description of the regulators

USB_VBUS is a controlled by a Silergy SY6288CCAC-GP 2A Power
Distribution Switch. The name of it's enable GPIO signal is USB_PWR_EN.

VCC5V is supplied by the main power input called PWR_5V_STB. The name of
it's enable GPIO signal is 3V3_5V_EN.

VCC3V3, VCC_DDR3_1V5 and VCCK (the CPU power supply) each use a separate
Silergy SY8089AAC-GP 2A step down regulator. They are all supplied by the
board's main 5V. VCC3V3 and VCC_DDR3_1V5 are fixed regulators while the
voltage of VCCK can be changed by changing it's feedback voltage via
PWM_C.

VCC1V8 is an ABLIC S-1339D18-M5001-GP fixed voltage regulator which is
supplied by VCC3V3.

VCC_RTC is a Global Mixed-mode Technology Inc. G918T12U-GP LDO which. It
is supplied by either VCC3V3 (when the board is powered) or the RTC coin
cell battery.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>