machine/boards: Fix-up naming of common dtsi files

* Common dtsi's did not match the expected naming convention, renamed
  them to match.
* zynq-7-base.dtsi -> zynq7-base.dtsi
* zynq-7-mainline-gem.dtsi -> zynq7-mainline-gem.dtsi
* Updated all inclusions of renamed files

Signed-off-by: Nathan Rossi <nathan.rossi@xilinx.com>
machine-xilinx-default.inc: Move base dts includes

* Move the base dts includes into the machine-xilinx-default.inc,
  removing the definition from the kernel recipes. This is to bring the
  device tree configuration inline with the kernel configuration
  variables.
* Fix the name of the dtsi, using the expected file paths which machine
  the naming convention.

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