conf/layer.conf: Add LAYERDEPENDS

* Add the LAYERDEPENDS variable to correctly define the layers
  meta-xilinx depends on. Currently only the core layer is a dependency

Signed-off-by: Nathan Rossi <nathan.rossi@xilinx.com>
zynq7-base.dtsi: Update bindings

* Update bindings for I2C
* Comment out non-secure DMA (PL330) node
* Add CPU voltage regulator node (fixed voltage, resolved 'failed to get
  cpu0 regulator error')

Signed-off-by: Nathan Rossi <nathan.rossi@xilinx.com>
zc70*-zynq7-board.dtsi: Update binding for si570

* Update the bindings for the si570 attached to the I2C bus

Signed-off-by: Nathan Rossi <nathan.rossi@xilinx.com>