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_defconfig b/configs/openpiton_riscv64_defconfig
index 1700f4a..7aec529 100644
--- a/configs/openpiton_riscv64_defconfig
+++ b/configs/openpiton_riscv64_defconfig
@@ -17,6 +17,7 @@
 CONFIG_USE_BOOTCOMMAND=y
 CONFIG_BOOTCOMMAND="fdt addr ${fdtcontroladdr}; fdt move ${fdtcontroladdr} ${fdt_addr_r}; load mmc ${mmcdev}:${mmcpart} ${kernel_addr_r} ${image}; booti ${kernel_addr_r} - ${fdt_addr_r}; "
 CONFIG_SYS_PROMPT="openpiton$ "
+CONFIG_SYS_PBSIZE=284
 # CONFIG_CMD_CPU is not set
 CONFIG_CMD_BOOTZ=y
 # CONFIG_BOOTM_NETBSD is not set