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/sipeed_maix_bitm_defconfig b/configs/sipeed_maix_bitm_defconfig
index 7d1722c..4a8f76e 100644
--- a/configs/sipeed_maix_bitm_defconfig
+++ b/configs/sipeed_maix_bitm_defconfig
@@ -11,6 +11,7 @@
 CONFIG_BOOTCOMMAND="run k210_bootcmd"
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_PBSIZE=276
 CONFIG_MTDIDS_DEFAULT="nor0=spi3:0"
 CONFIG_MTDPARTS_DEFAULT="nor0:1M(u-boot),0x1000@0xfff000(env)"
 # CONFIG_NET is not set