ARM: dts: sunxi: Fix PMU compatible strings

"arm,cortex-a15-pmu" is not a valid fallback compatible string for an
Cortex-A7 PMU, so drop it.

Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>