hw/net/lan9118: Add symbolic constants for register offsets

The lan9118 code mostly uses symbolic constants for register offsets;
the exceptions are those which the datasheet doesn't give an official
symbolic name to.

Add some names for the registers which don't already have them, based
on the longer names they are given in the memory map.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20210108180401.2263-3-peter.maydell@linaro.org