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/mt8183_pumpkin_defconfig b/configs/mt8183_pumpkin_defconfig
index ea7ab5c..95486ae 100644
--- a/configs/mt8183_pumpkin_defconfig
+++ b/configs/mt8183_pumpkin_defconfig
@@ -23,6 +23,7 @@
 CONFIG_DEFAULT_FDT_FILE="mt8183-pumpkin"
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_PBSIZE=276
 # CONFIG_CMD_BDI is not set
 # CONFIG_CMD_CONSOLE is not set
 # CONFIG_CMD_BOOTD is not set