ARM: vt8500: Add missing NULL terminator in dt_compat

When I tried booting a stih415 Dual core A9 with multi_v7_defconfig, it
failed to boot. The issues seems to be changing by enabling or disabling
VT8550 platform. Having a quick look at dt_compat list, it seems to miss
a NULL terminator, which means of_flat_dt_match will compat check will
cross the boundary of dt_compat and fault at some point , which is what
was happening in my case.

Without this patch if we try to boot multi_v7_defconfig you might notice
that some of the platforms might fault if they fall after vt8500 in
machine-desc list. Other platforms which fall before vt8500 in mdesc list
will not fault.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@st.com>