efi/libstub: Add definitions for console input and events

Add the required typedefs etc for using con_in's simple text input
protocol, and for using the boottime event services.

Also add the prototype for the "stall" boot service.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Link: https://lore.kernel.org/r/20200518190716.751506-19-nivedita@alum.mit.edu
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

arch/x86/xen/efi.c

@@ -29,7 +29,7 @@ static efi_system_table_t efi_systab_xen __initdata = {
 	.fw_vendor	= EFI_INVALID_TABLE_ADDR, /* Initialized later. */
 	.fw_revision	= 0,			  /* Initialized later. */
 	.con_in_handle	= EFI_INVALID_TABLE_ADDR, /* Not used under Xen. */
-	.con_in		= EFI_INVALID_TABLE_ADDR, /* Not used under Xen. */
+	.con_in		= NULL,			  /* Not used under Xen. */
 	.con_out_handle	= EFI_INVALID_TABLE_ADDR, /* Not used under Xen. */
 	.con_out	= NULL, 		  /* Not used under Xen. */
 	.stderr_handle	= EFI_INVALID_TABLE_ADDR, /* Not used under Xen. */