of: fix how an initrd is passed to Linux

Linux expects linux,initrd-end to contain the first unused address. As
this doesn't match the end semantic used by barebox (i.e. end contains
the last used address) adding one is necessary.

Without this change Linux fails for me to correctly extract a gzipped
cpio archive provided as initrd.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>