ARM: dts: meson8b: add the Mali-450 MP2 GPU

Add the Mali-450 GPU and it's OPP table for Meson8. The GPU uses two
pixel processors in this configuration. The OPP table is taken from the
3.10 vendor kernel which uses the following table:
  FCLK_DEV5 | 1,     /* 255 Mhz */
  FCLK_DEV7 | 0,     /* 364 Mhz */
  FCLK_DEV3 | 1,     /* 425 Mhz */
  FCLK_DEV5 | 0,     /* 510 Mhz */
  FCLK_DEV4 | 0,     /* 637.5 Mhz */
This describes the mux (FCLK_DEVx) and a 0-based divider in the clock
controller. "FCLK" is "fixed_pll" which is running at 2550MHz.
The "turbo" setting is described by "turbo_clock = 4" where 4 is the
index of the table above.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>