Use readb() and memcpy_fromio() accessors instead.

Ported from radeon commit:
f2c9e560Acked-by: NChristian König <christian.koenig@amd.com>
Acked-by: NJammy Zhou <Jammy.Zhou@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>