drivers/soc/litex: move generic accessors to litex.h

Move generic LiteX CSR (MMIO) register accessors to litex.h and
declare them as "static inline", in preparation for supporting
32-bit CSR subregisters and 64-bit CPUs.

NOTE: this is a non-functional change.

Signed-off-by: Gabriel Somlo <gsomlo@gmail.com>
Signed-off-by: Stafford Horne <shorne@gmail.com>