Set sctlr_el2 to predefined state for all CPUs

Currently, sctlr_el2 is initialised only in the cold boot path, and even then
we didn't set the RES1 bits. So we're lucky the cold boot path ever worked
given most of the bits are UNKNOWN.

Lack of initialisation in the hot boot path leads to kernel crash while CPU is
hot-plugging and KVM is enabled:

root@genericarmv8:~# echo 0 > /sys/devices/system/cpu/cpu1/online
kvm: disabling virtualization on CPU1
CPU1: shutdown
root@genericarmv8:~# echo 1 > /sys/devices/system/cpu/cpu1/online
Kernel panic - not syncing: HYP panic:
PS:000003c9 PC:0000000080002394 ESR:0000000086000005
FAR:0000000080002394 HPFAR:          (null) PAR:          (null)
VCPU:          (null)

CPU: 1 PID: 0 Comm: swapper/1 Not tainted 3.16.0-rc1+ #162
Call trace:
[<ffffffc0000880d8>] dump_backtrace+0x0/0x12c
[<ffffffc000088214>] show_stack+0x10/0x1c
[<ffffffc000472680>] dump_stack+0x74/0xc4
[<ffffffc00046f8ec>] panic+0xe4/0x21c
[<ffffffc00046f804>] mmu_memory_cache_alloc.part.25+0x34/0x38
[<ffffffc00008a26c>] cpu_psci_cpu_die+0x20/0x40
[<ffffffc00008e95c>] cpu_die+0x40/0x70
[<ffffffc0000852e0>] arch_cpu_idle_dead+0x8/0x14
[<ffffffc0000dca4c>] cpu_startup_entry+0x144/0x14c
[<ffffffc00008e7f0>] secondary_start_kernel+0x118/0x128

Initialise sctlr_el2 in the reset value just before dropping from EL3 for psci
and spin-table boot protocols.

Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>