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/evb-ast2600_defconfig b/configs/evb-ast2600_defconfig
index f84b723..6e5c1bd 100644
--- a/configs/evb-ast2600_defconfig
+++ b/configs/evb-ast2600_defconfig
@@ -40,6 +40,7 @@
 CONFIG_SPL_RAM_SUPPORT=y
 CONFIG_SPL_RAM_DEVICE=y
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_GPIO=y
 CONFIG_CMD_I2C=y
 CONFIG_CMD_MMC=y