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/kontron-sl-mx8mm_defconfig b/configs/kontron-sl-mx8mm_defconfig
index 2e9d525..7a3a8d5 100644
--- a/configs/kontron-sl-mx8mm_defconfig
+++ b/configs/kontron-sl-mx8mm_defconfig
@@ -35,6 +35,7 @@
 CONFIG_SPL_POWER=y
 CONFIG_SPL_WATCHDOG=y
 CONFIG_SPL_ATF=y
+CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_NVEDIT_EFI=y
 # CONFIG_CMD_LZMADEC is not set
 CONFIG_CMD_CLK=y