platform/mellanox: mlxbf-pmc: mlxbf_pmc_event_list(): make size ptr optional

platform/mellanox: mlxbf-pmc: mlxbf_pmc_event_list(): make size ptr optional

The mlxbf_pmc_event_list() function returns a pointer to an array of
supported events and the array size. The array size is returned via
a pointer passed as an argument, which is mandatory.

However, we want to be able to use mlxbf_pmc_event_list() just to check
if a block name is implemented/supported. For this usage passing the size
argument is not necessary so let's make it optional.

Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/182de8ec6b9c33152f2ba6b248c35b0311abf5e4.1708635408.git.luizcap@redhat.com
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>