usb: kbd: Set a default polling mechanism for USB keyboard

The choice of "USB keyboard polling" cannot be optional as without
one mechanism being set, it just doesn't work. Set the default one
to CONFIG_SYS_USB_EVENT_POLL.

Fixes: ecad7051 ("configs: Migrate all of the existing USB symbols, except fastboot")
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>