Documentation/powerpc: fix malformed table in syscall64-abi

Fix malformed table warning in powerpc/syscall64-abi.rst by making
two tables and moving the headings.

Documentation/powerpc/syscall64-abi.rst:53: WARNING: Malformed table.
Text in column margin in table line 2.

  =========== ============= ========================================
  --- For the sc instruction, differences with the ELF ABI ---
  r0          Volatile      (System call number.)
  r3          Volatile      (Parameter 1, and return value.)
  r4-r8       Volatile      (Parameters 2-6.)
  cr0         Volatile      (cr0.SO is the return error condition.)
  cr1, cr5-7  Nonvolatile
  lr          Nonvolatile

  --- For the scv 0 instruction, differences with the ELF ABI ---
  r0          Volatile      (System call number.)
  r3          Volatile      (Parameter 1, and return value.)
  r4-r8       Volatile      (Parameters 2-6.)
  =========== ============= ========================================

Fixes: 7fa95f9ada ("powerpc/64s: system call support for scv/rfscv instructions")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/e06de4d3-a36f-2745-9775-467e125436cc@infradead.org

Documentation/powerpc/syscall64-abi.rst

@@ -49,16 +49,18 @@ Register preservation rules
 Register preservation rules match the ELF ABI calling sequence with the
 following differences:
 
-=========== ============= ========================================
 --- For the sc instruction, differences with the ELF ABI ---
+=========== ============= ========================================
 r0          Volatile      (System call number.)
 r3          Volatile      (Parameter 1, and return value.)
 r4-r8       Volatile      (Parameters 2-6.)
 cr0         Volatile      (cr0.SO is the return error condition.)
 cr1, cr5-7  Nonvolatile
 lr          Nonvolatile
+=========== ============= ========================================
 
 --- For the scv 0 instruction, differences with the ELF ABI ---
+=========== ============= ========================================
 r0          Volatile      (System call number.)
 r3          Volatile      (Parameter 1, and return value.)
 r4-r8       Volatile      (Parameters 2-6.)