drivers: add RISC-V IMSIC interrupt driver

The RISC-V Advanced Interrupt Architecture (AIA) specification introduces
the IMSIC as a new external interrupt controller. An IMSIC receives and
records i

drivers: add RISC-V IMSIC interrupt driver

The RISC-V Advanced Interrupt Architecture (AIA) specification introduces
the IMSIC as a new external interrupt controller. An IMSIC receives and
records incoming message-signaled interrupts (MSIs).

This commit enables the initialization of the IMSIC based on the device
tree and adds control and status registers (CSRs) for indirect access to
the IMSIC as well as for reading interrupt identities.

Use the `CFG_RISCV_IMSIC` flag to control whether to build this driver.

For more details, see: https://github.com/riscv/riscv-aia

Signed-off-by: Huang Borong <huangborong@bosc.ac.cn>
Reviewed-by: Alvin Chang <alvinga@andestech.com>
Acked-by: Etienne Carriere <etienne.carriere@foss.st.com>

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