RAS: Export helper to get ras_debugfs_dir

Export a getter instead of the debugfs node directly so that, other
in-tree-only RAS modules can use it.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Yazen Ghannam <yazen.ghannam@amd.com>
Link: https://lore.kernel.org/r/20240301143748.854090-2-yazen.ghannam@amd.com