rockchip: dts: rk3399-puma: refactor and rename (default) DDR3-1600 DTS

rockchip: dts: rk3399-puma: refactor and rename (default) DDR3-1600 DTS

To better support different RAM timings (DDR3-1333 and DDR3-1866 are
assembly options for the RK3399-Q7), this refactors the DTS support
and renames the default DTS variant from rk3399-puma to
rk3399-puma-ddr1600:
- changes the rk3399-puma DTS into a board-specific DTSI by removing
  the inclusion of the DRAM timings
- adds a new rk3399-puma-ddr1600.dts, which includes the (new) common
  board DTSI and the DDR3-1600 timing DTSI
- wires this up from arch/arm/dts/Makefile and configs/puma-rk3399_defconfig

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Simon Glass <sjg@chromium.org>