[ioapi] Move get_memmap() to the I/O API group

pcbios specific get_memmap() is used by the b44 driver making
all-drivers builds fail on other platforms.  Move it to the I/O API
group and provide a dummy implementation on EFI.

Signed-off-by: Piotr Jaroszyński <p.jaroszynski@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/tests/umalloc_test.c

@@ -1,7 +1,7 @@
 #include <stdio.h>
 #include <ipxe/uaccess.h>
 #include <ipxe/umalloc.h>
-#include <ipxe/memmap.h>
+#include <ipxe/io.h>
 
 void umalloc_test ( void ) {
 	struct memory_map memmap;