ARM: dts: bcm2711-rpi-4-b: Adjust CMA size due to the OOM issue

ARM: dts: bcm2711-rpi-4-b: Adjust CMA size due to the OOM issue

Since 32-bit kernel has a limitation in the lowmem size, big CMA size
severely affect on available memory for kernel. Because of that, some test
cases make OOM kills due to the exhaustion in kernel available memory that
was reported by Jaehoon Chung (jh80.chung@samsung.com).

To handle this, I adjusted CMA size to 400MB while it meets the requirement
for UHD resolution.

Change-Id: I189699d4ce5f2866c01edf68c8138e9278679188
Signed-off-by: Sung-hun Kim <sfoon.kim@samsung.com>