arch: split dynamic interrupt symbol

This commit introduces the SRAM_SW_ISR_TABLE option which is selected by
DYNAMIC_INTERRUPT. It allows splitting the DYNAMIC_INTERRUPT option into
two parts:

 - One for the relocation of the ISR vector table in RAM
 - One for the inclusion of functions needed to install ISRs dynamically

The goal is to later only select the relocation of the ISR vector table in
RAM and not all the associated functions from the dynamic interrupt
mechanism.

Signed-off-by: Martin Hoff <martin.hoff@silabs.com>