riscv: cpu: nx25: Rename as ax25

Andes has rearranged the product combinations.
nx25 and ax25 both are RISC-V architecture cpu core.
But ax25 has MMU unit inside, and nx25 is not.

Cpu nx25 and platform ae250 are arranged in pairs.
Cpu ax25 and platform ae350 are arranged in pairs.

This patch will rename
nx25 as ax25
ae250 as ae350
nx25-ae250 as ax25-ae350
including filename, variable, string and definition.

Then u-boot can boot linux kernel in ae350
platform reasonably.

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