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/microchip_mpfs_icicle_defconfig b/configs/microchip_mpfs_icicle_defconfig
index 1a524a9..5453af8 100644
--- a/configs/microchip_mpfs_icicle_defconfig
+++ b/configs/microchip_mpfs_icicle_defconfig
@@ -12,6 +12,7 @@
 CONFIG_DISPLAY_CPUINFO=y
 CONFIG_DISPLAY_BOARDINFO=y
 CONFIG_SYS_PROMPT="RISC-V # "
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=282
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_BOOTP_SEND_HOSTNAME=y