arm: add secondary cpu boot callbacks to arm_boot.c

arm: add secondary cpu boot callbacks to arm_boot.c

Create two functions, write_secondary_boot() and secondary_cpu_reset_hook(),
to allow platforms more control of how secondary CPUs are brought up. The
new functions default to NULL and aren't called unless they are populated
so there are no changes to existing platform models.

Signed-off-by: Mark Langsdorf <mark.langsdorf@calxeda.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>