asix-sigma: rephrase and extend register access for readability

Reduce the probability of errors during maintenance, and also increase
readability. Replace open coded nibble extraction and bit positions by
accessor helpers and symbolic identifiers. Adjust existing math where it
did not match the vendor documentation. Always communicate 8bit register
addresses, don't assume that application use remains within a specific
"page". Provide more FPGA register access primitives so that call sites
need not re-invent FPGA command sequence construction. Remove remaining
open coded endianess conversion in DRAM access.