tests: kernel: mem_protect: Increase stack size for 64BIT plat

Some 64 bit platforms do not have MMU, however the 64 bit platfoms need
a larger stack. This testcase fails with non-MMU 64 bit platform e.g.
v8r64 platform due to the stack overflow. To fix this issue, set 2k
stack for all 64 bit platforms (CONFIG_64BIT) as this is likely a common
issue for all non-MMU 64 bit platforms.

Signed-off-by: Jaxson Han <jaxson.han@arm.com>