drivers: interrupt_controller: intc_shared: convert to multi instance

The intc_shared driver until now had the possibility to enable
up to two instances of the driver using the symbols SHARED_IRQ_0
and SHARED_IRQ_1.
This commit removes those Config options, and instead instantiates the
driver using DT_INST_FOREACH_STATUS_OKAY macro.

Signed-off-by: Thomas Stranger <thomas.stranger@outlook.com>