raspberrypi4-64.conf: Remove memory limitation

We can do this because the kernel now includes a workaround for the DMA
issue.
https://github.com/raspberrypi/linux/pull/3080

Signed-off-by: Andrei Gherzan <andrei@gherzan.ro>

conf/machine/raspberrypi4-64.conf

@@ -31,4 +31,4 @@ KERNEL_IMAGETYPE_UBOOT ?= "Image"
 KERNEL_IMAGETYPE_DIRECT ?= "Image"
 KERNEL_BOOTCMD ?= "booti"
 
-RPI_EXTRA_CONFIG ?= "\n# RPi4 64bit has some limitation - see https://github.com/raspberrypi/linux/commit/cdb78ce891f6c6367a69c0a46b5779a58164bd4b\ntotal_mem=1024\narm_64bit=1"
+RPI_EXTRA_CONFIG ?= "\n# Force arm in 64bit mode. See: https://github.com/raspberrypi/firmware/issues/1193.\narm_64bit=1"