arm64: dts: allwinner: a64: Drop PMU node

Looks like PMU in A64 is broken, it generates no interrupts at all and
as result 'perf top' shows no events.

Tested on Pine64-LTS.

Fixes: 34a97fcc ("arm64: dts: allwinner: a64: Add PMU node")
Cc: Harald Geyer <harald@ccbib.org>
Cc: Jared D. McNeill <jmcneill@NetBSD.org>
Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Reviewed-by: Emmanuel Vadot <manu@FreeBSD.org>
Signed-off-by: Maxime Ripard <mripard@kernel.org>