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/pico-imx7d_bl33_defconfig b/configs/pico-imx7d_bl33_defconfig
index 90355db..18fa002 100644
--- a/configs/pico-imx7d_bl33_defconfig
+++ b/configs/pico-imx7d_bl33_defconfig
@@ -32,6 +32,7 @@
 CONFIG_SPL_USB_SDP_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_MAXARGS=32
+CONFIG_SYS_PBSIZE=532
 # CONFIG_CMD_BOOTD is not set
 CONFIG_CMD_BOOTZ=y
 CONFIG_CMD_SPL=y