clk: meson: g12a: Add support for NNA CLK source clocks

This adds the Neural Network Accelerator source clocks hierarchy, it's
2 simple composite clocks to feed the AXI interface and the Core of
the Neural Network Accelerator IP.

This IP is only present on the Amlogic SM1 SoCs family.

Signed-off-by: Dmitry Shmidt <dimitrysh@google.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://lore.kernel.org/r/20200610083012.5024-3-narmstrong@baylibre.com