drm/hisilicon/hibmc: Make CONFIG_DRM_HISI_HIBMC depend on ARM64

Hisilicon developed hibmc_drm for their arm64 based soc and did not
intend for this driver to be used on any other architecture than arm64.

Using it on amd64 leads to incorrect video modes being used, making
the screen unreadable, forcing users to manually blacklist the module
on the kernel command line to use the d-i server installer or any
graphical sessions.

Make CONFIG_DRM_HISI_HIBMC firmly depend on ARM64 to ensure it is not
built for other architectures.

Signed-off-by: Matthew Ruffell <matthew.ruffell@canonical.com>
Signed-off-by: Xinliang Liu <z.liuxinliang@hisilicon.com>