USB gadget i.MX1: remove dependency on ARCH_MXC

The ARCH_MX1 scheduled for removal. Instead, depend on ARCH_MXC
and make clear in the Kconfig text that only i.MX1 has this
hardware.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>