valgrind: Fix powerpc64le arch config name

buildroot references powerpc64 little endian as "powerpc64le" and not
"powerpc64el". The typo in the supported architectures list caused this
package to be unavailable for powerpc64 little endian.

Signed-off-by: Erico Nunes <erico.nunes@datacom.ind.br>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>