ARM: zynq: Support smp in thumb mode

Add .arm directive to headsmp.S to ensure that the
CPU starts in 32-bit ARM mode and the correct code
size is copied on smp bring-up.
This is related to the fix applied to SoCFPGA by
commit 5616f367


("ARM: SoCFPGA: Fix secondary CPU startup in thumb2 kernel")

Additionally, start secondary CPUs on secondary_startup_arm
to automatically switch from ARM to thumb on a thumb kernel

Signed-off-by: Luis Araneda <luaraneda@gmail.com>
Suggested-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>