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/mx53cx9020_defconfig b/configs/mx53cx9020_defconfig
index f79595d..8d5c76f 100644
--- a/configs/mx53cx9020_defconfig
+++ b/configs/mx53cx9020_defconfig
@@ -13,6 +13,7 @@
 CONFIG_DISTRO_DEFAULTS=y
 CONFIG_BOOTDELAY=1
 CONFIG_USE_PREBOOT=y
+CONFIG_SYS_PBSIZE=532
 CONFIG_CMD_MMC=y
 CONFIG_CMD_USB=y
 # CONFIG_CMD_SETEXPR is not set