arm64: Apply ARM64_ERRATUM_1188873 to Neoverse-N1

Neoverse-N1 is also affected by ARM64_ERRATUM_1188873, so let's
add it to the list of affected CPUs.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
[will: Update silicon-errata.txt]
Signed-off-by: Will Deacon <will.deacon@arm.com>