juno: Implement plat_get_syscnt_freq() function ...

In the Juno port, this function returns the first entry of the
frequency modes table from the memory mapped generic timer.

juno: Implement plat_report_exception() function ...

As for FVP platforms, Juno provides some LEDs that we can
use to report exceptions during the early boot code.

juno: Port mailbox initialisation ...

Note, on Juno mailboxes are 16 bytes because any bigger and they would
overlap the memory used for MHU payload data for SCP->AP transfers.

Signed-off-by: Jon Medhurst <tixy@linaro.org>

juno: Override platform_get_core_pos and platform_is_primary_cpu ...

Signed-off-by: Jon Medhurst <tixy@linaro.org>

juno: Update memory map and use generic MMU code ...

Signed-off-by: Jon Medhurst <tixy@linaro.org>

juno: Improve compliance to Linux coding style

juno: Create as a trimmed copy of plat/fvp ...

Signed-off-by: Jon Medhurst <tixy@linaro.org>