MX28: Cleanup mxsboot within make mrproper

Delete the "mxsboot" binary if make mrproper is called.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
CC: Albert Aribaud <albert.u.boot@aribaud.net>
Acked-by: Stefano Babic <sbabic@denx.de>