When a new ibm,pa/pi-features bit is introduced that is intended to
apply to existing systems and features, it may have an "inverted"
meaning (i.e., bit clear => feature available; bit set => unavailable).
Depending on the nature of the feature, this may give the best
backward compatibility result where old firmware will continue to
have that bit clear and therefore the feature available.

The 'invert' modifier presumably was introduced for this type of
feature bit. However it invert will set the feature if the bit is
clear, which prevents it being used in the situation where an old
CPU lacks a feature that a new CPU has, then a new firmware comes
out to disable that feature on the new CPU if the bit is set.
Adding an 'invert' entry for that feature would incorrectly enable
it for the old CPU.

So add a 'clear' modifier that clears the feature if the bit is set,
but it does not set the feature if the bit is clear. The feature
is expected to be set in the cpu table.

This replaces the 'invert' modifier, which is unused since commit
7d470345 ("powerpc/feature: Remove CPU_FTR_NODSISRALIGN").
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Tested-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240207035220.339726-1-npiggin@gmail.com