hw/riscv/aia: Configure stride for the M-mode IMSIC

riscv_create_aia() currently hard-codes the M-IMSIC at one 4 KiB page
per hart and gives callers no way to widen it.

Add an m_imsic_stride parameter that supplies the per hart byte
stride directly. The virt machine passes IMSIC_HART_SIZE(0)
(= 4 KiB), preserving its existing compact layout.

The parameter only changes how the slots are spaced, with the rest of
each slot reserved. This allows future platforms that have different
layouts to control the stride.

Reviewed-by: Philippe Mathieu-Daudé <philmd@mailo.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Message-ID: <20260630024952.1520546-6-joel@jms.id.au>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>