Add a small README with information on the generic ohci driver.

doc/README.generic_usb_ohci

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+Notes on the the generic USB-OHCI driver
+========================================
+
+This driver (drivers/usb_ohci.[ch]) is the result of the merge of
+various existing OHCI drivers that were basically identical beside
+cpu/board dependant initalization. This initalization has been moved
+into cpu/board directories and are called via the hooks below.
+
+Configuration options
+----------------------
+
+CONFIG_USB_OHCI_NEW: enable the new OHCI driver
+
+CFG_USB_OHCI_BOARD_INIT: call the board dependant hooks:
+
+  - extern int usb_board_init(void);
+  - extern int usb_board_stop(void);
+  - extern int usb_cpu_init_fail(void);
+
+CFG_USB_OHCI_CPU_INIT: call the cpu dependant hooks:
+
+  - extern int usb_cpu_init(void);
+  - extern int usb_cpu_stop(void);
+  - extern int usb_cpu_init_fail(void);
+
+CFG_USB_OHCI_REGS_BASE: defines the base address of the OHCI registers
+
+CFG_USB_OHCI_SLOT_NAME: slot name
+
+CFG_USB_OHCI_MAX_ROOT_PORTS: maximal number of ports of the root hub.
+
+
+Endianness issues
+------------------
+
+The LITTLEENDIAN #define determines if the 'swap_16' and 'swap_32'
+macros do byte swapping or not. But some cpus OHCI-controllers such as
+ppc4xx and mpc5xxx operate in little endian mode, so some extra ifdefs
+were necessary to make this work.
+
+
+
+