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_defconfig b/configs/stm32f746-disco_defconfig
index 64d7d18..0473ed6 100644
--- a/configs/stm32f746-disco_defconfig
+++ b/configs/stm32f746-disco_defconfig
@@ -19,6 +19,7 @@
 # CONFIG_DISPLAY_CPUINFO is not set
 CONFIG_BOARD_LATE_INIT=y
 CONFIG_SYS_PROMPT="U-Boot > "
+CONFIG_SYS_PBSIZE=1050
 CONFIG_CMD_GPT=y
 # CONFIG_RANDOM_UUID is not set
 CONFIG_CMD_MMC=y