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/imx8mn_var_som_defconfig b/configs/imx8mn_var_som_defconfig
index d4930b1..b46f192 100644
--- a/configs/imx8mn_var_som_defconfig
+++ b/configs/imx8mn_var_som_defconfig
@@ -36,6 +36,7 @@
 CONFIG_SPL_WATCHDOG=y
 CONFIG_SYS_PROMPT="> "
 CONFIG_SYS_MAXARGS=64
+CONFIG_SYS_PBSIZE=2067
 CONFIG_CMD_FUSE=y
 CONFIG_CMD_MMC=y
 CONFIG_CMD_USB=y