ARM: exynos_defconfig: Update EHCI config entry

Commit 29824c16 ("USB: host: Rename ehci-s5p to ehci-exynos")
renamed the config entry of EHCI host driver. Similar change needs
to be done in exynos_defconfig as well.

Signed-off-by: Tushar Behera <tushar.behera@linaro.org>
Reviewed-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>