efi_loader: add some description about UEFI secure boot

A small text in docs/uefi/uefi.rst was added to explain how we can
configure and utilise UEFI secure boot feature on U-Boot.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>