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/sama5d36ek_cmp_spiflash_defconfig b/configs/sama5d36ek_cmp_spiflash_defconfig
index 9fbed91..0305025 100644
--- a/configs/sama5d36ek_cmp_spiflash_defconfig
+++ b/configs/sama5d36ek_cmp_spiflash_defconfig
@@ -27,6 +27,7 @@
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMI is not set
 # CONFIG_CMD_LOADS is not set