spl: Convert CONFIG_SPL_SIZE_LIMIT to hex

This is currently a decimal value which is not as convenient or
meaningful. Also U-Boot tends to use hex everywhere.

Convert this option to hex and add a comment for the size_check macro.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Acked-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: correct the typo in the commit title]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/configs/evb-rk3288_defconfig b/configs/evb-rk3288_defconfig
index 7b0e908..043ee32 100644
--- a/configs/evb-rk3288_defconfig
+++ b/configs/evb-rk3288_defconfig
@@ -4,7 +4,7 @@
 CONFIG_ROCKCHIP_RK3288=y
 CONFIG_TARGET_EVB_RK3288=y
 CONFIG_NR_DRAM_BANKS=1
-CONFIG_SPL_SIZE_LIMIT=307200
+CONFIG_SPL_SIZE_LIMIT=0x4b000
 CONFIG_SPL_STACK_R_ADDR=0x80000
 CONFIG_DEBUG_UART_BASE=0xff690000
 CONFIG_DEBUG_UART_CLOCK=24000000