timer: litex_timer: Add and use register names

Adds addresses and names for individual CSR registers to device tree.
This way timer driver no longer depends on CSR data width being 8 bits.
Also when register names their number changes, then overlay generated by
LiteX will be incompatible with one defined here.
This should make finding breaking changes easier.

I also updated register names to those used in current LiteX and
appended `_ADDR` suffix to defines which lacked them.

Because register `total` was renamed to `value` and `update_total` to
`update_value` I updated variables accordingly as well.

Signed-off-by: Michal Sieron <msieron@internships.antmicro.com>