ARM: spear: move spear.h and misc_regs.h into plat-spear

The spear13xx version of spear.h is completely different from
the newly combined spear3xx/spear6xx version, but we can never
build ARMv5 and ARMv7 platforms together, so there is no
harm in putting all the contents into a single file and adding
appropriate ifdefs.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>