Convert CONFIG_SYS_PBSIZE to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_PBSIZE

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/variscite_dart6ul_defconfig b/configs/variscite_dart6ul_defconfig
index cf17f7e..7322b12 100644
--- a/configs/variscite_dart6ul_defconfig
+++ b/configs/variscite_dart6ul_defconfig
@@ -21,6 +21,7 @@
 CONFIG_SPL_USB_HOST=y
 CONFIG_SPL_WATCHDOG=y
 CONFIG_SYS_MAXARGS=32
+CONFIG_SYS_PBSIZE=532
 CONFIG_CMD_DM=y
 CONFIG_CMD_GPIO=y
 CONFIG_CMD_GPT=y