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/openpiton_riscv64_spl_defconfig b/configs/openpiton_riscv64_spl_defconfig
index 8a7e3cc..4f3080a 100644
--- a/configs/openpiton_riscv64_spl_defconfig
+++ b/configs/openpiton_riscv64_spl_defconfig
@@ -27,6 +27,7 @@
 CONFIG_SPL_FS_EXT4=y
 CONFIG_SPL_RTC=y
 CONFIG_SYS_PROMPT="openpiton$ "
+CONFIG_SYS_PBSIZE=284
 # CONFIG_CMD_CPU is not set
 CONFIG_CMD_BOOTZ=y
 # CONFIG_BOOTM_NETBSD is not set