xtensa: xtfpga: fix integer overflow in TASK_SIZE

This fixes the following warning when default memory region crosses
0x80000000:

  arch/xtensa/include/asm/processor.h:40:47: warning:
    integer overflow in expression [-Woverflow]
    #define TASK_SIZE (PLATFORM_DEFAULT_MEM_START + PLATFORM_DEFAULT_MEM_SIZE)
                                               ^
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>