ARM: SH-Mobile: sh73a0: Secondary CPUs handle own SCU flags

When booting secondary CPUs we have used the main CPU to set up the
Snoop Control Unit flags of these CPUs. It is a cleaner approach
if every CPU takes care of its own flags. We avoid the need for
locking and the program logic is more concise. With this patch the file
headsmp-sh73a0.S is added that contains a startup vector for secondary CPUs
that sets up its own SCU flags.
Further in sh73a0_smp_prepare_cpus() we can rely on the generic ARM helper
scu_power_mode(). This is possible as we don't cross borders anymore (every
CPU handles its own flags) and need no locking. So we can throw out the
needless function modify_scu_cpu_psr().

Signed-off-by: Bastian Hecht <hechtb+renesas@gmail.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>