riscv: Clear pending interrupts before enabling IPIs

On some platforms (k210), the previous stage bootloader may have not
cleared pending IPIs before transferring control to U-Boot. This can cause
race conditions, as multiple harts all attempt to initialize the IPI
controller at once. This patch clears IPIs before enabling them, ensuring
that only one hart modifies shared memory at once.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>