travis.yml: Support RISC-V 64-bit

Fix riscv: ax25-ae350 build fail problem
https://travis-ci.org/trini/u-boot/jobs/385147373



...
Building current source for 1 boards (1 thread, 2 jobs per thread)
     riscv:  +   ax25-ae350
+arch/riscv/cpu/ax25/start.S: Assembler messages:
+arch/riscv/cpu/ax25/start.S:48: Error: unrecognized opcode `sd a2,0(t0)'
+arch/riscv/cpu/ax25/start.S:112: Error: unrecognized opcode `ld t5,0(t0)'
...

After apply the commit
configs: ax25-ae350: Set 64-bit as default configuration

Toolchain shall be also setuped with 64-bit in .travis.yml.

Signed-off-by: Rick Chen <rick@andestech.com>
Signed-off-by: Rick Chen <rickchen36@gmail.com>
Reviewed-by: Chih-Mao Chen <cmchen@andestech.com>
Cc: Greentime Hu <green.hu@gmail.com>