ARM platforms: Allow platforms to define SDEI events

With this patch, ARM platforms are expected to define the macros
PLAT_ARM_SDEI_PRIVATE_EVENTS and PLAT_ARM_SDEI_SHARED_EVENTS as a list
of private and shared events, respectively. This allows for individual
platforms to define their own events.

Change-Id: I66851fdcbff83fd9568c2777ade9eb12df284b49
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>