refactor(el3_runtime): unify handle/enter_lower_el_async_ea

refactor(el3_runtime): unify handle/enter_lower_el_async_ea

handle_lower_el_async_ea and enter_lower_el_async_ea are same except for
saving x30 register, with previous patch x30 is now freed before calling
these function we don't need both of them.

This patch also unifies the naming convention, now we have 3 handlers
 - handle_lower_el_ea_esb
 - handle_lower_el_sync_ea
 - handle_lower_el_async_ea

Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: I63b584cf059bac80195aa334981d50fa6272cf49