bcm4908: add missing config symbol

Signed-off-by: David Bauer <[email protected]>

target/linux/bcm4908/config-5.4

@@ -39,6 +39,7 @@ CONFIG_B53=y
 # CONFIG_B53_MDIO_DRIVER is not set
 # CONFIG_B53_MMAP_DRIVER is not set
 # CONFIG_B53_SERDES is not set
+# CONFIG_B53_SPI_DRIVER is not set
 # CONFIG_B53_SRAB_DRIVER is not set
 CONFIG_BCM4908_ENET=y
 CONFIG_BCM7XXX_PHY=y