soc: riscv: litex-vexriscv: change CSR accessors

Do not assume `reg_size` to be a multiple of 4 and
divide it on each call, expect the caller to provide
an already divided value instead.

This change require the LiteX GPIO driver to be modified,
which will come in the following commit.

Signed-off-by: Pawel Czarnecki <pczarnecki@internships.antmicro.com>
Signed-off-by: Mateusz Holenko <mholenko@antmicro.com>