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/socfpga_agilex_defconfig b/configs/socfpga_agilex_defconfig
index c8ccb05..f356466 100644
--- a/configs/socfpga_agilex_defconfig
+++ b/configs/socfpga_agilex_defconfig
@@ -28,6 +28,7 @@
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="SOCFPGA_AGILEX # "
 CONFIG_SYS_MAXARGS=64
+CONFIG_SYS_PBSIZE=2082
 CONFIG_CMD_MEMTEST=y
 CONFIG_CMD_GPIO=y
 CONFIG_CMD_I2C=y