phy: mediatek: allow compile-testing the dsi phy

Randconfig builds show another broken dependency:

WARNING: unmet direct dependencies detected for PHY_MTK_MIPI_DSI
  Depends on [n]: ARCH_MEDIATEK [=n] && OF [=y]
  Selected by [m]:
  - DRM_MEDIATEK [=m] && HAS_IOMEM [=y] && DRM [=m] && (ARCH_MEDIATEK [=n] || ARM [=y] && COMPILE_TEST [=y]) && COMMON_CLK [=y] &&
HAVE_ARM_SMCCC [=y] && OF [=y] && MTK_MMSYS [=y]

This is similar to the hdmi driver I fixed earlier, and I guess the
common-clk bug would sooner or later also manifest here, so just use
the exact same solution I chose for the other driver, and hope that
any future drivers just copy it from here.

Fixes: 90f80d95 ("phy: mediatek: Move mtk_mipi_dsi_phy driver into drivers/phy/mediatek folder")
Fixes: f5f6e01f

 ("phy: mediatek: allow compile-testing the hdmi phy")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20210103135524.3678664-1-arnd@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>