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/sama5d2_xplained_mmc_defconfig b/configs/sama5d2_xplained_mmc_defconfig
index 255fb0f..c4e885c 100644
--- a/configs/sama5d2_xplained_mmc_defconfig
+++ b/configs/sama5d2_xplained_mmc_defconfig
@@ -34,6 +34,7 @@
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_MISC_INIT_R=y
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_BOOTZ=y
 CONFIG_CMD_DM=y
 # CONFIG_CMD_FLASH is not set