Rename ppc-spapr-uv-hcalls.txt to ppc-spapr-uv-hcalls.rst.

Signed-off-by: Leonardo Garcia <lagarcia@br.ibm.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <ea1554e3a02e093eaa64d1260faf01d525c9ecdc.1642446876.git.lagarcia@br.ibm.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>