Add missing cache information to the Rockchip RK3399 SoC dtsi.  The specified
values were derived by hand from the cache size specifications available from
the RK3399 datasheet;  for future reference, here's a brief summary:

  - Each Cortex-A72 core has 48 KB of L1 instruction cache and
    32 KB of L1 data cache available, four-way set associative
  - Each Cortex-A53 core core has 32 KB of instruction cache and
    32 KB of L1 data cache available, four-way set associative
  - The big (A72) cluster has 1 MB of unified L2 cache available
  - The little (A53) cluster has 512 KB of unified L2 cache available

This patch allows /proc/cpuinfo and lscpu(1) to display proper RK3399 cache
information, and it eliminates the following error in the kernel log:

  cacheinfo: Unable to detect cache hierarchy for CPU 0

While there, add a couple of somewhat useful comments, which may help a bit
anyone going through the RK3399 SoC dtsi.
Co-developed-by: Kyle Copperfield <kmcopper@danwin1210.me>
Signed-off-by: Kyle Copperfield <kmcopper@danwin1210.me>
Signed-off-by: Dragan Simic <dsimic@manjaro.org>
Link: https://lore.kernel.org/r/be3cbcae5c40fa72a52845d30dcc66c847a98cfa.1702616304.git.dsimic@manjaro.orgSigned-off-by: Heiko Stuebner <heiko@sntech.de>