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/sama5d3_xplained_nandflash_defconfig b/configs/sama5d3_xplained_nandflash_defconfig
index d8dd849..3d8b32e 100644
--- a/configs/sama5d3_xplained_nandflash_defconfig
+++ b/configs/sama5d3_xplained_nandflash_defconfig
@@ -33,6 +33,7 @@
 CONFIG_SPL_NAND_DRIVERS=y
 CONFIG_SPL_NAND_BASE=y
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_FLASH is not set
 # CONFIG_CMD_LOADS is not set