ARM: ux500: add missing ENDPROC to headsmp.S

Once the ENDPROC is in place, BSYM() in not longer necessary
to get correct pointer to u8500_secondary_startup().

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Cc: Srinidhi Kasagar <srinidhi.kasagar@stericsson.com>
Cc: Linus Walleij <linus.walleij@stericsson.com>