[AMDGPU] Update Call Convention docs for GFX90A

Document the CSR AGPRs for GFX90A.

Remove the TODO for gfx908, as the answer is that we don't mark any
AGPRs as callee-saved except for GFX90A, i.e. the docs as-is are correct
for gfx908.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D109009