docs/system: riscv: Document AIA options for virt machine

We have two new machine options "aia" and "aia-guests" available
for the RISC-V virt machine so let's document these options.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Signed-off-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Frank Chang <frank.chang@sifive.com>
Message-Id: <20220220085526.808674-5-anup@brainfault.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

docs/system/riscv/virt.rst

@@ -63,6 +63,22 @@ The following machine-specific options are supported:
   When this option is "on", ACLINT devices will be emulated instead of
   SiFive CLINT. When not specified, this option is assumed to be "off".
 
+- aia=[none|aplic|aplic-imsic]
+
+  This option allows selecting interrupt controller defined by the AIA
+  (advanced interrupt architecture) specification. The "aia=aplic" selects
+  APLIC (advanced platform level interrupt controller) to handle wired
+  interrupts whereas the "aia=aplic-imsic" selects APLIC and IMSIC (incoming
+  message signaled interrupt controller) to handle both wired interrupts and
+  MSIs. When not specified, this option is assumed to be "none" which selects
+  SiFive PLIC to handle wired interrupts.
+
+- aia-guests=nnn
+
+  The number of per-HART VS-level AIA IMSIC pages to be emulated for a guest
+  having AIA IMSIC (i.e. "aia=aplic-imsic" selected). When not specified,
+  the default number of per-HART VS-level AIA IMSIC pages is 0.
+
 Running Linux kernel
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