MAINTAINERS: update USB EHCI platform pattern

Commit f3bc64d6 ("USB: EHCI: DT support for generic bus glue")
removed the ehci-vt8500.c file, update the file pattern to include
ehci-platform.c.

Signed-off-by: Joe Perches <joe@perches.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>