msm: emac: Add interrupt coalescing support

Add support in driver to change interrupt coalescing times
for tx and rx interrupts using standard ethtool. Interrupt
coalescing helps in reducing the number of interrupts and thus
improving throughput.

Change-Id: I42c0056f7543474a9931ab84edf22394a7a6eb79
Signed-off-by: Niranjana Vishwanathapura <nvishwan@codeaurora.org>