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_ddr4_evk_defconfig b/configs/imx8mn_ddr4_evk_defconfig
index 9aa2757..ce5aa8d 100644
--- a/configs/imx8mn_ddr4_evk_defconfig
+++ b/configs/imx8mn_ddr4_evk_defconfig
@@ -33,6 +33,7 @@
 CONFIG_SPL_WATCHDOG=y
 CONFIG_SYS_PROMPT="u-boot=> "
 CONFIG_SYS_MAXARGS=64
+CONFIG_SYS_PBSIZE=2074
 # CONFIG_CMD_EXPORTENV is not set
 # CONFIG_CMD_IMPORTENV is not set
 CONFIG_CMD_ERASEENV=y