drivers/soc/litex: separate MMIO from subregister offset calculation

Separate MMIO (read/write) access into _[read|write]_litex_subregister()
static inline functions, leaving existing "READ|WRITE" macros to handle
calculation of the subregister offset only.

NOTE: this is a non-functional change.

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