- Interrupt configuration is a 2-bit field, so the field shift has to
    be double that of the bit number.

  - Interrupt configuration (level- or edge-trigger) is specified in the
    MSB of the field, not LSB.

Fixes applied to both GICv2 and GICv3 drivers.

Fixes ARM-software/tf-issues#570

Change-Id: Ia6ae6ed9ba9fb0e3eb0f921a833af48e365ba359
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>