I realized that Linux mint doesn't support wake on USB mouse / keyboard out of the box. so I created following file and it started supporting wake on USB devices again.
[01:35:11 harsshal@dell:~]$ cat /etc/udev/rules.d/90-keyboardwakeup.rules
#idVendor and idProduct taken from 'lsusb'
SUBSYSTEM=="usb", ATTRS{idVendor}=="0461", ATTRS{idProduct}=="4d15" RUN+="/bin/sh -c 'echo enabled > /sys$env{DEVPATH}/../power/wakeup'"
SUBSYSTEM=="usb", ATTRS{idVendor}=="413c", ATTRS{idProduct}=="2002" RUN+="/bin/sh -c 'echo enabled > /sys$env{DEVPATH}/../power/wakeup'"
SUBSYSTEM=="usb", ATTRS{idVendor}=="413c", ATTRS{idProduct}=="1002" RUN+="/bin/sh -c 'echo enabled > /sys$env{DEVPATH}/../power/wakeup'"
PS: When I rebooted machine, for some reason, my idProduct was changed. So if these don't work after reboot, make sure numbers are correct using "lsusb" again.