x86: efi-x86_app: Update MAINTAINERS

Previous rename of efi-x86 target missed the MAINTAINERS update,
which caused the buildman warnings:

  WARNING: no status info for 'efi-x86_app'
  WARNING: no maintainers for 'efi-x86_app'

This updates the board MAINTAINERS to reflect the up-to-date info.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

board/efi/efi-x86_app/MAINTAINERS

@@ -1,6 +1,6 @@
-EFI-X86 BOARD
+EFI-X86_APP BOARD
 M:	Simon Glass <sjg@chromium.org>
 S:	Maintained
-F:	board/efi/efi-x86/
-F:	include/configs/efi-x86.h
-F:	configs/efi-x86_defconfig
+F:	board/efi/efi-x86_app/
+F:	include/configs/efi-x86_app.h
+F:	configs/efi-x86_app_defconfig