serial: sh-sci: FIFO sizing helper consolidation.

This consolidates all of the TX/RX fill/room nonsense in to a single set
of fairly heavyweight definitions. The implementation goes in descending
order of complexity, testing the register map for capabilities until we
run out of options and do it the legacy SCI way. Masks are derived
directly from the per-port FIFO size, meaning that platforms with FIFO
sizes not matching the standard port types will still need to manually
fix them up.

This also fixes up a number of issues such as tx_empty being completely
bogus for SCI and IrDA ports, some ports using masks smaller or greater
than their FIFO size, and so forth.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>