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/bitmain_antminer_s9_defconfig b/configs/bitmain_antminer_s9_defconfig
index 0d4007d..6e88048 100644
--- a/configs/bitmain_antminer_s9_defconfig
+++ b/configs/bitmain_antminer_s9_defconfig
@@ -31,6 +31,7 @@
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="antminer> "
 CONFIG_SYS_MAXARGS=32
+CONFIG_SYS_PBSIZE=2075
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_ELF is not set
 # CONFIG_CMD_DM is not set