AArch64: get rid of EL2 trampoline

AArch64: get rid of EL2 trampoline

There is no harm in initialising the platform from EL3, so we move the
call into the boot_common path of CPU0, and take the opportunity to
rename ns_init_system to init_platform. Caches and MMU are now disabled
at EL3, and we can also move cache maintenance in the initial boot path
of each CPU. This allows us to get rid of the EL2 layer, which makes
AArch32 kernel support possible.

Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>