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/sifive_unmatched_defconfig b/configs/sifive_unmatched_defconfig
index 5d07084..c7999f1 100644
--- a/configs/sifive_unmatched_defconfig
+++ b/configs/sifive_unmatched_defconfig
@@ -29,6 +29,7 @@
 CONFIG_SPL_DM_SPI_FLASH=y
 CONFIG_SPL_DM_RESET=y
 CONFIG_SPL_SPI_LOAD=y
+CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_EEPROM=y
 CONFIG_CMD_MEMINFO=y
 CONFIG_CMD_PWM=y