efi_driver: don't bind internal block devices

UEFI block devices can either mirror U-Boot's internal devices or be
provided by an EFI application like iPXE.

When ConnectController() is invoked for the EFI_BLOCK_IO_PROTOCOL
interface for such an application provided device we create a virtual
U-Boot block device of type "efi_blk".

Currently we do not call ConnectController() when handles for U-Boot's
internal block devices are created. If an EFI application calls
ConnectController() for a handle relating to an internal block device,
we erroneously create an extra "efi_blk" block device.

E.g. the UEFI shell has a command 'connect -r' which calls
ConnectController() for all handles with device path protocol.

In the Supported() method of our EFI_DRIVER_BINDING_PROTOCOL return
EFI_UNSUPPORTED when dealing with an U-Boot internal device.

Reported-by: Etienne Carriere <etienne.carriere@linaro.org>
Fixes: commit 05ef48a2 ("efi_driver: EFI block driver")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Etienne Carriere <etienne.carriere@linaro.org>
Tested-by: Etienne Carriere <etienne.carriere@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>