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/stm32mp15_defconfig b/configs/stm32mp15_defconfig
index 02b37e1..c22db79 100644
--- a/configs/stm32mp15_defconfig
+++ b/configs/stm32mp15_defconfig
@@ -21,6 +21,7 @@
 CONFIG_BOOTCOMMAND="run bootcmd_stm32mp"
 CONFIG_FDT_SIMPLEFB=y
 CONFIG_SYS_PROMPT="STM32MP> "
+CONFIG_SYS_PBSIZE=1050
 CONFIG_CMD_ADTIMG=y
 CONFIG_CMD_ERASEENV=y
 CONFIG_CMD_NVEDIT_EFI=y