MdePkg: Label CreateEvent NotifyFunction and NotifyContext optional

MdePkg: Label CreateEvent NotifyFunction and NotifyContext optional

According to the UEFI 2.10 Specification, the EFI_BOOT_SERVICES_TABLE
CreateEvent function has the following signature:

typedef
EFI_STATUS
(EFIAPI *EFI_CREATE_EVENT) (
  IN UINT32           Type,
  IN EFI_TPL          NotifyTpl,
  IN EFI_EVENT_NOTIFY NotifyFunction, OPTIONAL
  IN VOID             *NotifyContext, OPTIONAL
  OUT EFI_EVENT       *Event
);

Fix the prototype in UefiSpec.h to match, by labeling the NotifyFunction
and NotifyContext parameters as OPTIONAL.

Signed-off-by: Rebecca Cran <rebecca@bsdio.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>