USB: Add platform device support for the ISP1760 USB chip

Currently, the driver only supports PCI and PPC_OF but there are
boards like ARM RealView where this is a platform device. The patch adds
the necessary functions and registration to the isp1760-if.c file and
modifies the corresponding Makefile and Kconfig to be able to use this
driver even if PCI and PPC_OF are not enabled.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Sebastian Siewior <bigeasy@linutronix.de>
Cc: Russell King <rmk@arm.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>