mainboard/emulation/qemu-aarch64: Update DRAM_SIZE_MB

DRAM_SIZE_MB should be the maximum size (255GiB / -m 261120M)
that’s possible with QEMU on AArch64 virt because it tries to search
the DRAM_SIZE_MB range to find the true memory size.

Signed-off-by: Asami Doi <d0iasm.pub@gmail.com>
Change-Id: Id479c0b18d1e1adceecdcca13e36119b95617e6d
Reviewed-on: https://review.coreboot.org/c/coreboot/+/35024
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Raul Rangel <rrangel@chromium.org>

src/mainboard/emulation/qemu-aarch64/Kconfig

@@ -46,6 +46,6 @@ config MAINBOARD_VENDOR
 
 config DRAM_SIZE_MB
 	int
-	default 1024
+	default 261120 # The maximum dram size is 255GiB.
 
 endif #  BOARD_EMULATION_QEMU_AARCH64