ARM/dts: prima2: add .dtsi for atlas6 and .dts for atla6-evb board

atlas6.dtsi is basically a copy of prima2.dtsi as most components are
compatible with prima2 except that:
1. node of l2 cache is deleted
2. node multimedia engine is deleted
3. node of sata is deleted
4. node of sdmmc4 is deleted
5. powervr is moved to "powervr,sgx510"
6. pinctrl is moved to atlas6 as pinmux layout has big changes in
   atlas6
7. clock is moved to atlas6 as clock layout has changes in atlas6

Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Jiansong Chen <Jiansong.Chen@csr.com>