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/apple_m1_defconfig b/configs/apple_m1_defconfig
index 886fc4a..6bba71c 100644
--- a/configs/apple_m1_defconfig
+++ b/configs/apple_m1_defconfig
@@ -6,6 +6,7 @@
 # CONFIG_DISPLAY_CPUINFO is not set
 CONFIG_DISPLAY_BOARDINFO_LATE=y
 CONFIG_BOARD_LATE_INIT=y
+CONFIG_SYS_PBSIZE=276
 # CONFIG_NET is not set
 CONFIG_APPLE_SPI_KEYB=y
 # CONFIG_MMC is not set