selftests/powerpc: Fix L1D flushing tests for Power10

The rfi_flush and entry_flush selftests work by using the PM_LD_MISS_L1
perf event to count L1D misses.  The value of this event has changed
over time:

- Power7 uses 0x400f0
- Power8 and Power9 use both 0x400f0 and 0x3e054
- Power10 uses only 0x3e054

Rather than relying on raw values, configure perf to count L1D read
misses in the most explicit way available.

This fixes the selftests to work on systems without 0x400f0 as
PM_LD_MISS_L1, and should change no behaviour for systems that the tests
already worked on.

The only potential downside is that referring to a specific perf event
requires PMU support implemented in the kernel for that platform.

Signed-off-by: Russell Currey <ruscur@russell.cc>
Acked-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210223070227.2916871-1-ruscur@russell.cc