block: Update BLK to be def_bool

At this point in the DM migration, all platforms enable DM. BLK requires
DM. Make BLK "def_bool y" in the cases it had been "default y" to make
this clearer. Now remove the symbol requirement from other places as it
is redundant here.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/drivers/block/Kconfig b/drivers/block/Kconfig
index 6ad1888..48529a6 100644
--- a/drivers/block/Kconfig
+++ b/drivers/block/Kconfig
@@ -1,8 +1,8 @@
 config BLK
 	bool # "Support block devices"
 	depends on DM
-	default y if MMC || USB || SCSI || NVME || IDE || AHCI || SATA
-	default y if EFI_MEDIA || VIRTIO_BLK || PVBLOCK
+	def_bool y if MMC || USB || SCSI || NVME || IDE || AHCI || SATA
+	def_bool y if EFI_MEDIA || VIRTIO_BLK || PVBLOCK
 	help
 	  Enable support for block devices, such as SCSI, MMC and USB
 	  flash sticks. These provide a block-level interface which permits