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/bananapi_m2_zero_defconfig b/configs/bananapi_m2_zero_defconfig
index ac3f8f5..74c164c 100644
--- a/configs/bananapi_m2_zero_defconfig
+++ b/configs/bananapi_m2_zero_defconfig
@@ -6,3 +6,4 @@
 CONFIG_DRAM_CLK=408
 CONFIG_MMC0_CD_PIN=""
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
+CONFIG_SYS_PBSIZE=1024