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/omap3_beagle_defconfig b/configs/omap3_beagle_defconfig
index 5c3c846..8a54276 100644
--- a/configs/omap3_beagle_defconfig
+++ b/configs/omap3_beagle_defconfig
@@ -23,6 +23,7 @@
 CONFIG_SPL_NAND_BASE=y
 CONFIG_SYS_PROMPT="BeagleBoard # "
 CONFIG_SYS_MAXARGS=64
+CONFIG_SYS_PBSIZE=1055
 CONFIG_CMD_SPL=y
 CONFIG_CMD_SPL_NAND_OFS=0x280000
 CONFIG_CMD_SPL_WRITE_SIZE=0x20000