doc: arch: Add document for RISC-V architecture

This patch adds a brief introduction to the RISC-V architecture and
the typical boot process used on a variety of RISC-V platforms.

Signed-off-by: Yu Chien Peter Lin <peterlin@andestech.com>
Reviewed-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Rick Chen <rick@andestech.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/MAINTAINERS b/MAINTAINERS
index 6f53f9c..41c9f26 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -1298,6 +1298,7 @@
 T:	git https://source.denx.de/u-boot/custodians/u-boot-riscv.git
 F:	arch/riscv/
 F:	cmd/riscv/
+F:	doc/arch/riscv.rst
 F:	doc/usage/sbi.rst
 F:	drivers/sysreset/sysreset_sbi.c
 F:	drivers/timer/andes_plmt_timer.c