cmd: load: add load command for memory mapped

cp.b is used a lot as a way to load binaries to memory and execute
them, however we may need to integrate this with the efi subsystem to
set it up as a bootdev.

So, introduce a loadm command that will be consistent with the other
loadX commands and will call the efi API's.

ex: loadm $kernel_addr $kernel_addr_r $kernel_size

with this a kernel with CONFIG_EFI_STUB enabled will be loaded and
then subsequently booted with bootefi command.

Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 7704184..8d08ea1 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -44,6 +44,7 @@
    cmd/fatload
    cmd/for
    cmd/load
+   cmd/loadm
    cmd/loady
    cmd/mbr
    cmd/md