Documentation: update arch list in the 'memtest' entry

Since arm64/arm support memtest command line option update the "memtest"
entry.

Change-Id: I5ccc820a8ae4375002b70c1fde1e1d314eb75149
Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Patch-mainline: linux-arm-kernel @ 03/09/15, 10:27
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>