fix(cpus): use hint instruction for "tsb csync"

fix(cpus): use hint instruction for "tsb csync"

The "tsb csync" instruction is part of the Armv8.4 architecture
extension, and is not supported by many older assemblers.
We already cater for this in lib/extensions/trbe/trbe.c, where we use
the equivalent "hint #18" encoding for this, but use the new mnemonic
in the Cortex-A510 CPU support code.

Replace "tsb csync" with the hint encoding there as well, to support
building with older binutils versions.

Change-Id: Idf39f5c6c4dbf72802c3c120047b8bc499145e3b
Signed-off-by: Andre Przywara <andre.przywara@arm.com>