Fix compile error related to r8a66597-hcd & usb

When building the 8544DS board we get this error:

In file included from r8a66597-hcd.c:22:
u-boot/include/usb.h:190:2: error: #error USB Lowlevel not defined
make[1]: *** [r8a66597-hcd.o] Error 1

The cleanest fix is to only build r8a66597-hcd.c if CONFIG_USB_R8A66597_HCD
is set.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>