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/xilinx_zynq_virt_defconfig b/configs/xilinx_zynq_virt_defconfig
index 75afd1c..61a748b 100644
--- a/configs/xilinx_zynq_virt_defconfig
+++ b/configs/xilinx_zynq_virt_defconfig
@@ -34,6 +34,7 @@
 CONFIG_SPL_SPI_LOAD=y
 CONFIG_SYS_SPI_U_BOOT_OFFS=0x100000
 CONFIG_SYS_MAXARGS=32
+CONFIG_SYS_PBSIZE=2071
 # CONFIG_BOOTM_NETBSD is not set
 CONFIG_CMD_IMLS=y
 CONFIG_CMD_THOR_DOWNLOAD=y