fix(rcar3): fix RPC-IF device node name

fix(rcar3): fix RPC-IF device node name

According to the Generic Names Recommendation in the Devicetree
Specification Release v0.3, and the DT Bindings for the Renesas Reduced
Pin Count Interface, the node name for a Renesas RPC-IF device should be
"spi".  The node name matters, as the node is enabled by passing a DT
fragment from TF-A to subsequent software.

Fix this by renaming the device node in the passed DT fragment from
"rpc" to "spi".

Fixes: 12c75c8886a0ee69 ("feat(plat/rcar3): emit RPC status to DT fragment if RPC unlocked")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Change-Id: Idb43353947607611331abc344f8c8ae932a20408