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/ls1021atwr_qspi_defconfig b/configs/ls1021atwr_qspi_defconfig
index 1edc123..08d0c00 100644
--- a/configs/ls1021atwr_qspi_defconfig
+++ b/configs/ls1021atwr_qspi_defconfig
@@ -32,6 +32,7 @@
 CONFIG_ARCH_MISC_INIT=y
 CONFIG_MISC_INIT_R=y
 CONFIG_ID_EEPROM=y
+CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_GREPENV=y
 CONFIG_SYS_EEPROM_PAGE_WRITE_DELAY_MS=5
 CONFIG_CMD_MEMINFO=y