Allow CSS to redefine function `plat_arm_calc_core_pos`

Allow CSS to redefine function `plat_arm_calc_core_pos`

Currently all ARM CSS platforms which include css_helpers.S use the same
strong definition of `plat_arm_calc_core_pos`. This patch allows these CSS
platforms to define their own strong definition of this function.

* Replace the strong definition of `plat_arm_calc_core_pos` in
  css_helpers.S with a utility function `css_calc_core_pos_swap_cluster`
  does the same thing (swaps cluster IDs). ARM CSS platforms may choose
  to use this function or not.

* Add a Juno strong definition of `plat_arm_calc_core_pos`, which uses
  `css_calc_core_pos_swap_cluster`.

Change-Id: Ib5385ed10e44adf6cd1398a93c25973eb3506d9d