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/ae350_rv32_spl_defconfig b/configs/ae350_rv32_spl_defconfig
index 8406eb5..502701f 100644
--- a/configs/ae350_rv32_spl_defconfig
+++ b/configs/ae350_rv32_spl_defconfig
@@ -17,6 +17,7 @@
 CONFIG_BOARD_EARLY_INIT_F=y
 # CONFIG_BINMAN_FDT is not set
 CONFIG_SYS_PROMPT="RISC-V # "
+CONFIG_SYS_PBSIZE=1050
 CONFIG_CMD_IMLS=y
 CONFIG_CMD_MMC=y
 CONFIG_CMD_SF_TEST=y