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/sama5d27_wlsom1_ek_qspiflash_defconfig b/configs/sama5d27_wlsom1_ek_qspiflash_defconfig
index 691b0c2..4cdc2b0 100644
--- a/configs/sama5d27_wlsom1_ek_qspiflash_defconfig
+++ b/configs/sama5d27_wlsom1_ek_qspiflash_defconfig
@@ -39,6 +39,7 @@
 CONFIG_SYS_SPI_U_BOOT_OFFS=0x40000
 CONFIG_SPL_AT91_MCK_BYPASS=y
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_BOOTZ=y
 CONFIG_CMD_DM=y
 # CONFIG_CMD_FLASH is not set