riscv: Add min dts support for miv SoC & m2gl025_miv board

Add minimal DTS support for the m2gl025_miv.  The dts just has the core,
SoC, PLIC, and UART described.  We still get memory related info from
Kconfig.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>