powerpc/powernv: Silence SYSPARAM warning on boot

OpenPower BMC machines do not place any sysparams in the device tree, so
at every boot we get a warning:

 [    0.437176] SYSPARAM: Opal sysparam node not found

Remove the warning, and reorder the init so we don't peform allocations
when there is no sysparam node in the device tree.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Acked-by: Neelesh Gupta <neelegup@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>