feat(plat/arm/css): add interrupt handler for reboot request

feat(plat/arm/css): add interrupt handler for reboot request

Add platform specific interrupt handler for handling the reboot of
all CPU's. On shutdown/reboot, only one CPU invoke PSCI and enter into
trusted firmware. The CPU which entered trusted firmware signals the
rest of the cores which are online using SGI to initiate power down
sequence. On receiving the SGI, the handler will power down the
GIC redistributor interface of the respective core, configure the power
control register and power down the CPU by executing wfi.

In addition to these changes, fix coding style issues that are not
directly related to the code being introduced in this patch.

Change-Id: I4917dfdc47be5ce7367bee629486a6344cdd706f
Signed-off-by: Pranav Madhu <pranav.madhu@arm.com>