USB: host: Rename ehci-s5p to ehci-exynos

Currently, Samsung is using 'EXYNOS' as the name of Samsung SoCs.
Thus, ehci-exynos is preferred than ehci-s5p.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>