[AMDGPU] Correct documentation for default setting of sram-ecc

[t-tye]

Summary

• Correct documentation for default setting of sram-ecc to ON for gfx906.

[t-tye]

Correct commit heading.

[kzhuravl]

Might want to mention that it is being corrected for gfx906 in the commit message

[kzhuravl]

sramecc is off by default for gfx906:
https://github.com/llvm/llvm-project/blob/master/llvm/test/CodeGen/AMDGPU/elf-header-flags-sram-ecc.ll#L5

[t-tye]

So it seems the e_flags report that sram-ecc off, yet the code generated is for on.

So we should probably wait for the Target ID changes that fix these multiple bugs.

[arsenm]

When I still had a working gfx906 it reported ECC as on

llvm/docs/AMDGPUUsage.rst

.. table:: AMDGPU Processors
      ``gfx904``                  ``amdgcn``   dGPU  - xnack                   *TBA*
                                                       [off]
                                                                               .. TODO::
                                                                                  Add product
                                                                                  names.
      ``gfx906``                  ``amdgcn``   dGPU  - xnack                   - Radeon Instinct MI50
                                                       [off]                   - Radeon Instinct MI60
                                                     - sram-ecc                - Radeon VII
-                                                      [off]                   - Radeon Pro VII
+                                                      [on]                    - Radeon Pro VII
      ``gfx908``                  ``amdgcn``   dGPU  - xnack                   *TBA*
                                                       [off]
                                                     - sram-ecc
                                                       [on]
                                                                               .. TODO::
                                                                                  Add product
                                                                                  names.
      ``gfx909``                  ``amdgcn``   APU   - xnack                   *TBA*