usb: xhci-msm-hsic: Add support to modify imod from sysfs

xhci allows interrupts to be moderated at 250ns interval;
provide an sysfs interface to modify interrupt rate from
userspace to understand throughput variation by changing imod.

Usage:
Below command will moderate interrupts at 1ms or (4000 * 250ns)
interval.
echo 4000 > /sys/devices/msm_hsic_host/config_imod

Change-Id: Ic308faa046135dec731ef7a1e00f659142758a73
Signed-off-by: Vamsi Krishna <vskrishn@codeaurora.org>