usb: musb: Kconfig: Drop dependency on MACH_JZ4740 for jz4740

Depending on MACH_JZ4740 prevent us from creating a generic kernel that
works on more than one MIPS board. Instead, we just depend on MIPS being
set.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>