efi_driver: EFI block driver

This patch provides
* a uclass for EFI drivers
* a EFI driver for block devices

For each EFI driver the uclass
* creates a handle
* adds the driver binding protocol

The uclass provides the bind, start, and stop entry points for the driver
binding protocol.

In bind() and stop() it checks if the controller implements the protocol
supported by the EFI driver. In the start() function it calls the bind()
function of the EFI driver. In the stop() function it destroys the child
controllers.

The EFI block driver binds to controllers implementing the block io
protocol.

When the bind function of the EFI block driver is called it creates a
new U-Boot block device. It installs child handles for all partitions and
installs the simple file protocol on these.

The read and write functions of the EFI block driver delegate calls to the
controller that it is bound to.

A usage example is as following:

U-Boot loads the iPXE snp.efi executable. iPXE connects an iSCSI drive and
exposes a handle with the block IO protocol. It calls ConnectController.

Now the EFI block driver installs the partitions with the simple file
protocol.

iPXE uses the simple file protocol to load Grub or the Linux Kernel.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
[agraf: add comment on calloc len]
Signed-off-by: Alexander Graf <agraf@suse.de>