serial: uart_liteuart: Add and use register names

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

I also appended `_ADDR` suffix to defines, to distinguish them from
normal values like `UART_EX_TX`.

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