efi: Add a media/block driver for EFI block devices

Add a block driver which handles read/write for EFI block devices. This
driver actually already exists ('efi_block') but is not really suitable
for use as a real U-Boot driver:

- The operations do not provide a udevice
- The code is designed for running as part of EFI loader, so uses
    EFI_PRINT() and EFI_CALL().
- The bind method probes the device, which is not permitted
- It uses 'EFI' as its parent device

The new driver is more 'normal', just requiring its platform data be set
up in advance.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>