drivers/soc/litex: s/LITEX_REG_SIZE/LITEX_SUBREG_ALIGN/g

The constant LITEX_REG_SIZE is renamed to the more descriptive
LITEX_SUBREG_ALIGN (LiteX CSR subregisters are located at 32-bit
aligned MMIO addresses).

NOTE: this is a non-functional change.

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