drm/amdgpu/soc15: fix xclk for raven

It's 25 Mhz (refclk / 4).  This fixes the interpretation
of the rlc clock counter.

Acked-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org