powerpc/85xx: Fix virt_to_phys() off-by-one in smp_85xx_start_cpu()

In smp_85xx_start_cpu() we are passed an address but we're unsure if
it's a real or virtual address, so there's a check to determine that.

The check has an off-by-one in that it tests if the address is greater
than high_memory, but high_memory is the first address of high memory,
so the check should be greater-or-equal.

It seems this has never been a problem in practice, but it also triggers
the DEBUG_VIRTUAL checks in __pa() which we would like to avoid. We can
fix both issues by converting high_memory - 1 to a physical address and
testing against that.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220406145802.538416-3-mpe@ellerman.id.au