feat(msm8916): allow booting secondary CPU cores

Add support for the PSCI CPU_ON call to allow booting secondary CPU
cores. On cold boot they need to be booted with a special register
sequence. Also, the "boot remapper" needs to be configured to point to
the BL31_BASE, so the CPUs actually start executing BL31 after reset.

Change-Id: I406c508070ccb046bfdefd51554f12e1db671fd4
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>