soc/mediatek/common/emi: Cache SDRAM size

Optimize the `sdram_size` function by caching the calculated SDRAM
size in a static variable. This prevents redundant calls to
`mtk_dram_size()` or `mem_chip_info_total_density_bytes()` if the size
has already been determined, improving performance in scenarios where
`sdram_size` is called multiple times.

BUG=none
TEST=emerege-tanjiro coreboot

Change-Id: I0ca0df80ee9cb781a5bb6d55ee28a2c1153be0ad
Signed-off-by: Yidi Lin <yidilin@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/90485
Reviewed-by: Yu-Ping Wu <yupingso@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Hung-Te Lin <hungte@chromium.org>

src/soc/mediatek/common/emi.c

@@ -8,7 +8,10 @@
 size_t sdram_size(void)
 {
 	const struct mem_chip_info *mc;
-	size_t size = 0;
+	static size_t size = 0;
+
+	if (size)
+		return size;
 
 	if (ENV_RAMINIT) {
 		size = mtk_dram_size();