- Set number of TX descriptors to 512.

- Set number of TX descriptors to 512.
- Add interrupt moderation support in msk(4).
- Add a sysctl node for interrupt rate, which is disabled by default. (*)

Obtained-from: Marvell's myk(4)

#
# Set interrupt rate to 4000(/sec) on Yukon EC(rev2):
# - If the other side sends at 200Kpps, interrupt time will be reduced from
#   50%-70% to 30%-50%
# - If msk(4) is used to blast packets, interrupt time will be reduced from
#   35%-40% to 25%-%35 and packet rate will be increased by 10Kpps-15Kpps
#

# (*) Marvell's myk(4) uses different clock for EC_U/FE/XL than the value set
#     in mskc_attach(), due to lack of document, it is unclear which clock is
#     correct, so disabling interrupt moderation timer by default is the safest
#     choice.