drm: exynos: dsi: Mark PHY as optional

The same Samsung MIPI DSIM master can also be used in NXP's
i.MX8M Mini/Nano/Plus SoC.

In i.MX8M Mini/Nano/Plus SoC the DSI Phy requires a MIPI DPHY
bit to reset in order to activate the PHY and that can be done
via upstream i.MX8M blk-ctrl driver.

So, mark the phy get as optional.

Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>