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/am3517_evm_defconfig b/configs/am3517_evm_defconfig
index 184e886..26468ca 100644
--- a/configs/am3517_evm_defconfig
+++ b/configs/am3517_evm_defconfig
@@ -31,6 +31,7 @@
 # CONFIG_SPL_POWER is not set
 CONFIG_SYS_PROMPT="AM3517_EVM # "
 CONFIG_SYS_MAXARGS=64
+CONFIG_SYS_PBSIZE=1054
 # CONFIG_CMD_IMI is not set
 CONFIG_CMD_SPL=y
 CONFIG_CMD_SPL_NAND_OFS=0xaa0000