x86/tdx: Make macros of TDCALLs consistent with the spec

The TDX spec names all TDCALLs with prefix "TDG".  Currently, the kernel
doesn't follow such convention for the macros of those TDCALLs but uses
prefix "TDX_" for all of them.  Although it's arguable whether the TDX
spec names those TDCALLs properly, it's better for the kernel to follow
the spec when naming those macros.

Change all macros of TDCALLs to make them consistent with the spec.  As
a bonus, they get distinguished easily from the host-side SEAMCALLs,
which all have prefix "TDH".

No functional change intended.

Signed-off-by: Kai Huang <kai.huang@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/all/516dccd0bd8fb9a0b6af30d25bb2d971aa03d598.1692096753.git.kai.huang%40intel.com