ARM: dts: meson8: remove two invalid interrupt lines from the GPU node

The 3.10 vendor kernel defines the following GPU 20 interrupt lines:
  #define INT_MALI_GP                 AM_IRQ(160)
  #define INT_MALI_GP_MMU             AM_IRQ(161)
  #define INT_MALI_PP                 AM_IRQ(162)
  #define INT_MALI_PMU                AM_IRQ(163)
  #define INT_MALI_PP0                AM_IRQ(164)
  #define INT_MALI_PP0_MMU            AM_IRQ(165)
  #define INT_MALI_PP1                AM_IRQ(166)
  #define INT_MALI_PP1_MMU            AM_IRQ(167)
  #define INT_MALI_PP2                AM_IRQ(168)
  #define INT_MALI_PP2_MMU            AM_IRQ(169)
  #define INT_MALI_PP3                AM_IRQ(170)
  #define INT_MALI_PP3_MMU            AM_IRQ(171)
  #define INT_MALI_PP4                AM_IRQ(172)
  #define INT_MALI_PP4_MMU            AM_IRQ(173)
  #define INT_MALI_PP5                AM_IRQ(174)
  #define INT_MALI_PP5_MMU            AM_IRQ(175)
  #define INT_MALI_PP6                AM_IRQ(176)
  #define INT_MALI_PP6_MMU            AM_IRQ(177)
  #define INT_MALI_PP7                AM_IRQ(178)
  #define INT_MALI_PP7_MMU            AM_IRQ(179)

However, the driver from the 3.10 vendor kernel does not use the
following four interrupt lines:
- INT_MALI_PP3
- INT_MALI_PP3_MMU
- INT_MALI_PP7
- INT_MALI_PP7_MMU

Drop the "pp3" and "ppmmu3" interrupt lines. This is also important
because there is no matching entry in interrupt-names for it (meaning
the "pp2" interrupt is actually assigned to the "pp3" interrupt line).

Fixes: 7d3f6b53

 ("ARM: dts: meson8: add the Mali-450 MP6 GPU")
Reported-by: Thomas Graichen <thomas.graichen@gmail.com>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Tested-by: thomas graichen <thomas.graichen@gmail.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://lore.kernel.org/r/20200815181957.408649-1-martin.blumenstingl@googlemail.com