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/rock960-rk3399_defconfig b/configs/rock960-rk3399_defconfig
index be0e1c7..ce37b57 100644
--- a/configs/rock960-rk3399_defconfig
+++ b/configs/rock960-rk3399_defconfig
@@ -20,6 +20,7 @@
 CONFIG_SPL_STACK_R_MALLOC_SIMPLE_LEN=0x10000
 CONFIG_TPL=y
 CONFIG_SYS_PROMPT="rock960 => "
+CONFIG_SYS_PBSIZE=1052
 CONFIG_CMD_BOOTZ=y
 CONFIG_CMD_GPT=y
 CONFIG_CMD_MMC=y