ARM: zx: fix building with CONFIG_THUMB2_KERNEL

ARM: zx: fix building with CONFIG_THUMB2_KERNEL

The newly added zx platform causes a build error when
CONFIG_THUMB2_KERNEL is enabled:

arch/arm/mach-zx/headsmp.S:16: Error: invalid immediate for address calculation (value = 0x00000004)

I'm assuming that the ROM code that is calling these entry
points runs in ARM mode, so there would be another problem
in the same file, and we can solve both problems at once
by adding a '.arm' statement that will make zx_resume_jump
and zx_secondary_startup both be built as ARM code.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Jun Nie <jun.nie@linaro.org>
Tested-by: Jun Nie <jun.nie@linaro.org>