sh: r2dplus: Add pci enum to preboot

The ethernet on r2dplus is on PCI bus, enumerate the PCI early
to get the ethernet operational.
Signed-off-by: NMarek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>

configs/r2dplus_defconfig

@@ -8,6 +8,8 @@ CONFIG_TARGET_R2DPLUS=y
 CONFIG_BOOTDELAY=-1
 CONFIG_USE_BOOTARGS=y
 CONFIG_BOOTARGS="console=ttySC0,115200"
+CONFIG_USE_PREBOOT=y
+CONFIG_PREBOOT="pci enum"
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_IMLS=y
 CONFIG_CMD_DM=y