drivers: intc_dw: support multiple instances

Update the initializers to support platforms with multiple instances
of the DesignWare interrupt aggregator. Also ensure that the initialize
function calls irq_enable(). The calculation of the _sw_isr_table entry
also now takes CONFIG_GEN_IRQ_START_VECTOR into account.

Signed-off-by: Aaron Fong <afong@tenstorrent.com>