arm: bcm7xxx: Convert to DM_SERIAL

Remove ns16550 configuration from header files.  Document
DM_SERIAL-required prior stage device tree configuration.
diff --git a/configs/bcm7260_defconfig b/configs/bcm7260_defconfig
index f8e0327..51e0a5d 100644
--- a/configs/bcm7260_defconfig
+++ b/configs/bcm7260_defconfig
@@ -40,6 +40,6 @@
 CONFIG_MMC_SDHCI=y
 CONFIG_MMC_SDHCI_BCMSTB=y
 CONFIG_MTD=y
-CONFIG_SYS_NS16550_SERIAL=y
-CONFIG_SYS_NS16550_REG_SIZE=-4
+CONFIG_DM_SERIAL=y
+CONFIG_SYS_NS16550=y
 # CONFIG_EFI_LOADER is not set