soc: riscv: litex: Add helpers for accessing CSRs

Depending on LiteX configuration, CSRs
(control&status registers) might be split
into several consecutive registers.

This introduces common helper functions
for all LiteX drivers providing access
to CSRs for a default LiteX configuration
(data_width = 8bit, bus_width = 32bit).

Signed-off-by: Mateusz Holenko <mholenko@antmicro.com>