i2c: rcar_i2c: Add comments about registers & values

Document the meaning of macros related to registers and values to be
written to them.

Signed-off-by: Ismael Luceno <ismael.luceno@silicon-gears.com>
Reviewed-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
1 file changed