USB: move common alchemy USB routines to arch/mips/alchemy/common.c

A previous patch converted the Alchemy platform to use the OHCI and EHCI
platform drivers. As a result, all the common logic to handle USB present in
drivers/usb/host/alchemy-common.c has no reason to remain here, so we move it
to arch/mips/alchemy/common/usb.c which is a more appropriate place. This
change was suggested by Manuel Lauss.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>