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>