dwc3: core: added an API to invoke irq handlers

Since interrupt support is not present in u-boot, added an
API to handle the interrupts in dwc3 core. This API can be
polled to handle the interrupts.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Reviewed-by: Lukasz Majewski <l.majewski@samsung.com>