drivers/intel: Align FSP debug handler with EFI calling convention

Ensures the FSP debug handler adheres to the EFI calling convention,
enabling seamless integration with coreboot infrastructure.

This is critical for 64-bit coreboot and FSP communications.

BUG=b:242829490
TEST=FSP debug logs successfully captured via coreboot event handler.

Change-Id: I9085a6c7d50e58fb56cbbc61da3a0af094d0dc05
Signed-off-by: Subrata Banik <subratabanik@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/81621
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Dinesh Gehlot <digehlot@google.com>
Reviewed-by: Ronak Kanabar <ronak.kanabar@intel.com>

src/drivers/intel/fsp2_0/fsp_debug_event.c

@@ -17,7 +17,7 @@ static efi_return_status_t print_fsp_string_data(const efi_status_code_data_t *d
 	return FSP_SUCCESS;
 }
 
-efi_return_status_t fsp_debug_event_handler(efi_status_code_type_t ignored1,
+__efiapi efi_return_status_t fsp_debug_event_handler(efi_status_code_type_t ignored1,
 	efi_status_code_value_t ignored2, efi_uint32_t ignored3, efi_guid_t *ignored4,
 	efi_status_code_data_t *data)
 {

src/drivers/intel/fsp2_0/include/fsp/fsp_debug_event.h

@@ -12,7 +12,7 @@
 #include <fsp/soc_binding.h>
 
 /* fsp debug event handler */
-efi_return_status_t fsp_debug_event_handler(efi_status_code_type_t ignored1,
+__efiapi efi_return_status_t fsp_debug_event_handler(efi_status_code_type_t ignored1,
 	efi_status_code_value_t ignored2, efi_uint32_t ignored3, efi_guid_t *ignored4,
 	efi_status_code_data_t *data);