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/q8_a13_tablet_defconfig b/configs/q8_a13_tablet_defconfig
index f269b8a..08be881 100644
--- a/configs/q8_a13_tablet_defconfig
+++ b/configs/q8_a13_tablet_defconfig
@@ -16,6 +16,7 @@
 CONFIG_VIDEO_LCD_BL_PWM="PB2"
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
 CONFIG_SPL_I2C=y
+CONFIG_SYS_PBSIZE=1024
 CONFIG_SYS_I2C_MVTWSI=y
 CONFIG_SYS_I2C_SLAVE=0x7f
 CONFIG_SYS_I2C_SPEED=400000