cmd: efidebug: add dh command

"dh" command prints all the uefi handles used in the system.

=> efi dh
        7ef3bfa0: Device Path, Device Path To Text, Device Path Utilities,
		  Unicode Collation 2
        7ef31d30: Driver Binding
        7ef31da0: Simple Text Output
        7ef31e10: Simple Text Input, Simple Text Input Ex
        7ef3cca0: Block IO, Device Path
        7ef3d070: Block IO, Device Path
        7ef3d1b0: Block IO, Device Path, Simple File System
        7ef3d3e0: Block IO, Device Path, Simple File System

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>