Convert CONFIG_SYS_CBSIZE to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_CBSIZE

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/xilinx_zynqmp_r5_defconfig b/configs/xilinx_zynqmp_r5_defconfig
index 1aed730..1ae6349 100644
--- a/configs/xilinx_zynqmp_r5_defconfig
+++ b/configs/xilinx_zynqmp_r5_defconfig
@@ -16,6 +16,7 @@
 # CONFIG_DISPLAY_CPUINFO is not set
 CONFIG_SYS_PROMPT="ZynqMP r5> "
 CONFIG_SYS_MAXARGS=32
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=284
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_CMD_BOOTSTAGE=y