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/ls1021aqds_qspi_defconfig b/configs/ls1021aqds_qspi_defconfig
index d677ff7..a7f5cb0 100644
--- a/configs/ls1021aqds_qspi_defconfig
+++ b/configs/ls1021aqds_qspi_defconfig
@@ -31,6 +31,7 @@
 CONFIG_MISC_INIT_R=y
 CONFIG_ID_EEPROM=y
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_BOOTZ=y
 CONFIG_CMD_GREPENV=y
 CONFIG_SYS_EEPROM_PAGE_WRITE_DELAY_MS=5