drm/amd/include: adjust base offset of SMUIO and THM for Arcturus

Arcturus has different _BASE_IDX value in some HWIP_offset.h. To make source
files like smu_v11_0.c and soc15.c that include HWIP_offset.h of Vega20
reusable for Arcturus, align this base offset with Vega20.

Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Kenneth Feng <kenneth.feng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>