arm: socfpga: Add missing ',' in CONFIG_BOOTARGS

Somehow the sr1500 is missing this comma in the CONFIG_BOOTARGS
definition. This patch adds it to.

Signed-off-by: Stefan Roese <sr@denx.de>
Reported-by: Pavel Machek <pavel@denx.de>
Cc: Pavel Machek <pavel@denx.de>
Cc: Marek Vasut <marex@denx.de>