riscv: t-head: licheepi4a: initial support added

Add support for Sipeed's Lichee Pi 4A board which based on T-HEAD's
TH1520 SoC, only minimal device tree and serial console are enabled,
so it's capable of chain booting from T-HEAD's vendor u-boot.

Reviewed-by: Wei Fu <wefu@redhat.com>
Signed-off-by: Yixun Lan <dlan@gentoo.org>