include/efi: Introduce __efiapi for EFI calling convention flexibility

This patch defines __efiapi (based on EFIAPI) for coreboot-compliant
EFI calls. This lays the groundwork for future 64-bit EFI calling
convention support within coreboot/FSP.

BUG=b:242829490
TEST=FSP debug log accessible via coreboot event handler.

Change-Id: I21660f8ebeed3b9ef060118928a940a470492bb8
Signed-off-by: Subrata Banik <subratabanik@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/81620
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Dinesh Gehlot <digehlot@google.com>
Reviewed-by: Varshit Pandya <pandyavarshit@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/include/efi/efi_datatype.h

@@ -34,6 +34,9 @@ typedef BMP_IMAGE_HEADER efi_bmp_image_header;
 typedef BMP_COLOR_MAP efi_bmp_color_map;
 #endif
 
+/* EFIAPI calling convention */
+#define __efiapi EFIAPI
+
 /* Basic Data types */
 /* 8-byte unsigned value. */
 typedef UINT64 efi_uint64_t;