bmips: allow disabling usb support

bmips: allow disabling usb support

Currently, if usb is disabled the following error is produced:
  CC      drivers/usb/host/ohci-hcd.o
drivers/usb/host/ohci-hcd.c: In function ‘usb_lowlevel_init’:
drivers/usb/host/ohci-hcd.c:2057:35: error: ‘CONFIG_SYS_USB_OHCI_REGS_BASE’ undeclared (first use in this function); did you mean ‘CONFIG_SYS_MONITOR_BASE’?
  gohci.regs = (struct ohci_regs *)CONFIG_SYS_USB_OHCI_REGS_BASE;
                                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                   CONFIG_SYS_MONITOR_BASE
drivers/usb/host/ohci-hcd.c:2057:35: note: each undeclared identifier is reported only once for each function it appears in
drivers/usb/host/ohci-hcd.c:2061:20: error: ‘CONFIG_SYS_USB_OHCI_SLOT_NAME’ undeclared (first use in this function); did you mean ‘CONFIG_SYS_USB_OHCI_MAX_ROOT_PORTS’?
  gohci.slot_name = CONFIG_SYS_USB_OHCI_SLOT_NAME;
                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                    CONFIG_SYS_USB_OHCI_MAX_ROOT_PORTS

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>