efi_loader: add missing const classifier for event service

const classifier is missing in EventGroup parameter of
CreateEventEx(). Fix it to remove the compiler warning.

NotifyContext parameter of CreateEventEx() is also defined
with const in UEFI specification, but NotifyContext parameter
of CreateEvent() is defined without const.
Since current implementation calls the common efi_create_event()
function from both CreateEventEx() and CreateEvent() services,
NotifyContext parameter leaves as is.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>