tools: mkeficapsule: add man page

Add a man page for mkeficapsule command.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
diff --git a/MAINTAINERS b/MAINTAINERS
index b6a4b5d..1c98029 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -762,6 +762,7 @@
 T:	git https://source.denx.de/u-boot/custodians/u-boot-efi.git
 F:	doc/api/efi.rst
 F:	doc/develop/uefi/*
+F:	doc/mkeficapsule.1
 F:	doc/usage/bootefi.rst
 F:	drivers/rtc/emul_rtc.c
 F:	include/capitalization.h