ARM: multi_v7_defconfig: make Samsung Exynos EHCI driver a module

Exynos EHCI driver is compiled as kernel built-in, but it requires Samsung
USB2 Generic PHY driver to operate properly, which is compiled as module.
Make the Exynos EHCI driver also a module, because having it built-in
makes no sense. Exynos OHCI, which also uses that PHY driver, is already
compiled as a module.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Link: https://lore.kernel.org/r/20201124083312.12356-1-m.szyprowski@samsung.com
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>