arm64: Call swiotlb_init() instead of swiotlb_init_with_default_size()

Following commit 74838b75 (swiotlb: add the late swiotlb initialization
function with iotlb memory) the swiotlb_init_with_default_size() is a
static function. This patch changes the arm64 code to call
swiotlb_init() instead and use the default size of 64MB. It is assumed
that AArch64 platforms have enough RAM to afford the pre-allocated
swiotlb memory. It also removes the #ifdef around this call since
CONFIG_SWIOTLB is always enabled.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>