ARM: dts: imx51: Add tigerp support

As per the i.MX51 Reference Manual add an entry for the
'tigerp' region in the device tree.

This is needed for accessing the ARM_GPC register to set the
DBGEN bit, so that the debug clocks can be turned on.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>