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/pomelo_defconfig b/configs/pomelo_defconfig
index 5dcd1e5..008ea76 100644
--- a/configs/pomelo_defconfig
+++ b/configs/pomelo_defconfig
@@ -14,6 +14,7 @@
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_LAST_STAGE_INIT=y
 CONFIG_SYS_PROMPT="pomelo#"
+CONFIG_SYS_PBSIZE=280
 CONFIG_OF_CONTROL=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_DM_PCI_COMPAT=y