valgrind: use the BR2_ARM_CPU_ARM* options

Instead of using the exhaustive list of Cortex-A ARM cores, use
BR2_ARM_CPU_ARMV7A instead. We also fix a wrong comment that said the
Valgrind package could only be selected for Cortex-A8 and Cortex-A9,
which was true a long time ago, but was no longer true since it was
also enabled for all the other Cortex-A platforms.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>