usb: musb: name ux500 platforms more broadly

The Kconfig help text is talking about the U5500 which is no
longer supported by the kernel. Name the help text after the
config symbol which is more correct.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>