powerpc/powernv: Simplify searching for compatible device nodes

This condenses the opal node searching into a single function that finds
all compatible nodes, instead of just searching the ibm,opal children,
for ipmi, flash, and prd similar to how opal-i2c nodes are found.

Signed-off-by: Jack Miller <jack@codezen.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>