soc: litex: Add function for reading 64bit CSRs This adds a helper function for reading LiteX 64-bit CSRs to be used by LiteX drivers. Signed-off-by: Jakub Cebulski <jcebulski@internships.antmicro.com> Signed-off-by: Mateusz Holenko <mholenko@antmicro.com>