toolchain: add arm toolchain for xiaomi router r1d

R1D use the BCM4709 SoC with two ARM Cortex A9 cpu core

Cortex A9 is ARMv7 architecture and the implemetation of
BCM4709 is without neon and vfp. So the key parameters of
toolchain is:

 --with-arch=armv7-a
 --with-tune=cortex-a9
 --with-float=soft (no vfp)

no neon, little endian, eabi

Signed-off-by: Jack Tan <jiankemeng@gmail.com>