efi_loader: Add disk interfaces

A EFI applications usually want to access storage devices to load data from.

This patch adds support for EFI disk interfaces. It loops through all block
storage interfaces known to U-Boot and creates an EFI object for each existing
one. EFI applications can then through these objects call U-Boot's read and
write functions.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>
[trini: Update for various DM changes since posting]
Signed-off-by: Tom Rini <trini@konsulko.com>