arm64: timer: add TVAL accessors

The ARM arch timer features the CVAL register, which holds an absolute
value that is compared against the counter value.
But there is also the TVAL register, which is defined as (CVAL - counter),
and can be used to program or read relative timeouts.

Add accessors for the TVAL register, to be able to read and write them
easily later.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>