ARM: owl: smp: Drop bogus holding pen

The S500 SoC can start secondary CPUs without busy-looping for pen_release,
so simplify the SMP code compared to the LeMaker kernel tree.

Fixes: 172067e0 ("ARM: owl: Implement CPU enable-method for S500")
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Cc: David Liu <liuwei@actions-semi.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>