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/MPC8548CDS_defconfig b/configs/MPC8548CDS_defconfig
index ee9c148..55acf63 100644
--- a/configs/MPC8548CDS_defconfig
+++ b/configs/MPC8548CDS_defconfig
@@ -19,6 +19,7 @@
 # CONFIG_MISC_INIT_R is not set
 CONFIG_ID_EEPROM=y
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_IMLS=y
 CONFIG_SYS_I2C_EEPROM_ADDR_LEN=2
 CONFIG_CMD_I2C=y