Platform/ARM/N1Sdp: Modify the IRQ ID of Debug UART and routing it to IOFPGA UART1

In DBG2 table, IRQ ID was set as 0 for the UART. This overwrote the
IPI0 trigger method to "level", which prevented SGI0 to be enabled
again after a CPU offline/online cycle.

This patch fixes the above issue by assigning a reserved IRQ ID
for the Debug UART, other than 0 and also routing it to use IOFPGA
UART1 by unsharing it from currently using serial terminal.

Signed-off-by: Himanshu Sharma <Himanshu.Sharma@arm.com>
Change-Id: I6640c3c8f77afd233304ce9cb06dcf80a8659c16