amdkfd: add __iomem attribute to doorbell_ptr

This patch was done due to sparse warning. It changes the definition of
doorbell_ptr in queue_properties to be with __iomem attribute, so it would
match the type which the doorbell module functions are returning.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Oded Gabbay <oded.gabbay@amd.com>