docs: powerpc: Fix tables in syscall64-abi.rst

docs: powerpc: Fix tables in syscall64-abi.rst

Commit 209b44c804c ("docs: powerpc: syscall64-abi.rst: fix a malformed
table") attempted to fix the formatting of tables in syscall64-abi.rst, but
inadvertently changed some register names.

Redo the tables with the correct register names, and while we're here,
clean things up to separate the registers into different rows and add
headings.

Fixes: 209b44c804c ("docs: powerpc: syscall64-abi.rst: fix a malformed table")
Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>
Link: https://lore.kernel.org/r/20210225060857.16083-1-ajd@linux.ibm.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>