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/stm32f746-disco_spl_defconfig b/configs/stm32f746-disco_spl_defconfig
index c417a4f..440022d 100644
--- a/configs/stm32f746-disco_spl_defconfig
+++ b/configs/stm32f746-disco_spl_defconfig
@@ -32,6 +32,7 @@
 CONFIG_SPL_XIP_SUPPORT=y
 CONFIG_SPL_DM_RESET=y
 CONFIG_SYS_PROMPT="U-Boot > "
+CONFIG_SYS_PBSIZE=1050
 CONFIG_CMD_GPT=y
 # CONFIG_RANDOM_UUID is not set
 CONFIG_CMD_MMC=y