Escape minus signs in manpage

By default, "-" chars are interpreted as hyphens (U+2010) by groff, not
as minus signs (U+002D). Since options to programs use minus signs
(U+002D), this means for example in UTF-8 locales that you cannot cut
and paste options, nor search for them easily.

(Reported by lintian.)
Signed-off-by: NLoc Minier <loic.minier@linaro.org>

doc/mkimage.1

@@ -41,22 +41,22 @@ mkimage lists the information contained in the header of an existing U-Boot imag
 
 .TP
 .BI "\-A [" "architecture" "]"
-Set architecture. Pass -h as the architecture to see the list of supported architectures.
+Set architecture. Pass \-h as the architecture to see the list of supported architectures.
 
 .TP
 .BI "\-O [" "os" "]"
 Set operating system. bootm command of u-boot changes boot method by os type.
-Pass -h as the OS to see the list of supported OS.
+Pass \-h as the OS to see the list of supported OS.
 
 .TP
 .BI "\-T [" "image type" "]"
 Set image type.
-Pass -h as the image to see the list of supported image type.
+Pass \-h as the image to see the list of supported image type.
 
 .TP
 .BI "\-C [" "compression type" "]"
 Set compression type.
-Pass -h as the compression to see the list of supported compression type.
+Pass \-h as the compression to see the list of supported compression type.
 
 .TP
 .BI "\-a [" "load addess" "]"