[CORE] vmm_devtree: Allow node attributes with NULL value

Node attributes with NULL value can be very handy to represent
boolean FDT attributes. In other words, for boolean node attributes
we add attribute with NULL value to indicate that attribute is
available and we delete the attribtue to indicate that attribute
is not available.

Signed-off-by: Anup Patel <anup@brainfault.org>