soc: litex-vexriscv: Implement universal LiteX HAL

Adds LITEX_CSR_DATA_WIDTH option to Kconfig
Depending on its value appropriate read/write handling is used
for accessing CSR registers.
By using `>=` in preprocessor conditions it is somewhat future-proofed.

Doesn't touch `litex_read` and `litex_write` yet.

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