ARM: sunxi: fix build for THUMB2_KERNEL

Building an SMP kernel for the sunxi platform with THUMB2 instructions
fails with this error at the moment:

headsmp.S:7: Error: Thumb encoding does not support an immediate here -- `msr cpsr_fsxc,#0xd3'

Since the generic secondary_startup function already does
the same thing in a safe way, we can just drop the private
sunxi implementation and jump straight to secondary_startup.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Maxime Ripard <maxime.ripard@free-electrons.com>