cpu_has_feature()/mmu_has_feature() are only able to check a single
feature at a time, but there is no enforcement of that.

In fact, as fixed in the previous commit, there was code that was
passing multiple values to cpu_has_feature().

So add a check that only a single feature is passed using popcount.

Note that the test allows 0 or 1 bits to be set, because some code
relies on cpu_has_feature(0) being false, the check with
CPU_FTRS_POSSIBLE ensures that. See for example CPU_FTR_PPC_LE.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240509121248.270878-3-mpe@ellerman.id.au