bcm4908: fix backport of PMB driver

Missing Makefile change was preventing kernel from actually compiling
the driver.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>

target/linux/bcm4908/patches-5.4/082-v5.12-0002-soc-bcm-add-PM-driver-for-Broadcom-s-PMB.patch

@@ -404,3 +404,11 @@ Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
  source "drivers/soc/bcm/brcmstb/Kconfig"
  
  endmenu
+--- a/drivers/soc/bcm/Makefile
++++ b/drivers/soc/bcm/Makefile
+@@ -1,4 +1,5 @@
+ # SPDX-License-Identifier: GPL-2.0-only
+ obj-$(CONFIG_BCM2835_POWER)	+= bcm2835-power.o
+ obj-$(CONFIG_RASPBERRYPI_POWER)	+= raspberrypi-power.o
++obj-y				+= bcm63xx/
+ obj-$(CONFIG_SOC_BRCMSTB)	+= brcmstb/