SPEAr : usbd driver support for SPEAr SoCs

SPEAr SoCs contain a synopsys usb device controller.
USB Device IP can work in 2 modes
- DMA mode
- Slave mode

The driver adds support only for slave mode operation of usb
device IP. This driver is used along with standard USBTTY
driver to obtain a tty interface over USB on the host

Signed-off-by: Vipin <vipin.kumar@st.com>