arm64: decrustify and extend SMP boot code

The SMP boot code depends on physical CPU #0 to be first to boot and
subsequent CPUs to follow suit in a linear fashion. Let's decouple
physical and logical numbering so that any physical CPU can be the
boot CPU. This is based on a prior code proposal from
Jiafei Pan <Jiafei.Pan@nxp.com>.

This, however, was about to turn the boot code into some hairy mess.
So let's clean things up and simplify the code as well while at it.
Both the extension and the clean up aren't separate commits because
they actually depend on each other.

The BOOT_PARAM_*_OFFSET defines are locally hardcoded as there is no
point exposing the related structure widely. Build time assertions
ensure they don't go out of sync with the struct definition. And
vector_table.h is repurposed into boot.h to gather boot related
definitions.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
Signed-off-by: Jiafei Pan <Jiafei.Pan@nxp.com>