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/myir_mys_6ulx_defconfig b/configs/myir_mys_6ulx_defconfig
index d647f1c..f5ccf66 100644
--- a/configs/myir_mys_6ulx_defconfig
+++ b/configs/myir_mys_6ulx_defconfig
@@ -25,6 +25,7 @@
 CONFIG_SPL_USB_GADGET=y
 CONFIG_SPL_WATCHDOG=y
 CONFIG_SYS_MAXARGS=32
+CONFIG_SYS_PBSIZE=532
 CONFIG_CMD_MEMTEST=y
 CONFIG_CMD_DM=y
 CONFIG_CMD_GPIO=y