clk: meson: meson8b: add the CPU clock post divider clocks

There are four CPU clock post dividers:
- ABP
- PERIPH (used for the ARM global timer and ARM TWD timer)
- AXI
- L2 DRAM

Each of these clocks consists of two clocks:
- a mux to select between "cpu_clk" divided by 2, 3, 4, 5, 6, 7 or 8
- a "_clk_dis" gate. The public S805 datasheet states that this should
  be set to 1 to disable the clock, the default value is 0. There is
  also a hint that these are "just in case" bits which only exist in
  case the corresponding mux implementation does not allow glitch-free
  parent changes (the muxes are designed in a way that the clock can
  stay enabled when changing the mux). It's still good practise to
  describe this clock even if we're not supposed to modify it. Thus
  this uses the read-only gate ops.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://lkml.kernel.org/r/20181122214017.25643-5-martin.blumenstingl@googlemail.com