arm64: defconfig: change VA bits to 39 from 48

VA bits of 48 gives more virtual address space for virtual machine.
This much of virtual space is not required for virtual machine.
Configure less virtual address that is of 39 bits.

Change-Id: Iac343d8c6dd1667abfedc1613aaa4f517008ac83
Signed-off-by: Murali Nalajala <mnalajal@codeaurora.org>