ARM: dts: imx51: Add aipstz nodes

Currently when trying to use the 'devmem' tool to read/write
registers the following bus error is observed:

[   82.689185] Unhandled fault: external abort on non-linefetch (0x1018) at 0xb6ff10c4
[   82.697071] pgd = (ptrval)
[   82.699920] [b6ff10c4] *pgd=a87c5831, *pte=73fa8383, *ppte=73fa8a33
Bus error

Add the aipstz nodes, so that the following function from
imx51_dt_init() can work properly:

	imx_aips_allow_unprivileged_access("fsl,imx51-aipstz");

After adding these nodes, 'devmem' tool can work correctly,
which is very useful for debugging purposes.

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