sys: util: use BITS_PER_BYTE macro instead of the magic number 8

Obviously, everyone knows that there are 8 bits per byte, so
there isn't a lot of magic happening, per se, but it's also
helpful to clearly denote where the magic number 8 is referring
to the number of bits in a byte.

Occasionally, 8 will refer to a field size or offset in a
structure, MMR, or word. Occasionally, the number 8 will refer
to the number of bytes in a 64-bit value (which should probably
be replaced with `sizeof(uint64_t)`).

For converting bits to bytes, or vice-versa, let's use
`BITS_PER_BYTE` for clarity (or other appropriate `BITS_PER_*`
macros).

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>