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/imx8mp_evk_defconfig b/configs/imx8mp_evk_defconfig
index 90a6f39..3921650 100644
--- a/configs/imx8mp_evk_defconfig
+++ b/configs/imx8mp_evk_defconfig
@@ -38,6 +38,7 @@
 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_CRC32 is not set