cmake: gen_isr_tables: Reduce verbosity of non-verbose builds.
cmake: gen_isr_tables: Reduce verbosity of non-verbose builds.

gen_isr_tables.py's --debug flag causes it to spit out a verbose
log. Normal builds don't need to see this information.

This patch will remove this from the build log:

gen_isr_tables.py: (4041, 4477, 46, 0, 7)
gen_isr_tables.py: spurious handler: 0xfc9
gen_isr_tables.py: Configured interrupt routing
gen_isr_tables.py: handler    irq flags param
gen_isr_tables.py: --------------------------
gen_isr_tables.py: 0x14bd     0   0     0x0
gen_isr_tables.py: 0x16a1     6   0     0x20002bac
gen_isr_tables.py: 0x1975     17  0     0x0
gen_isr_tables.py: 0x7e5b     13  0     0x0
gen_isr_tables.py: 0x7e55     24  0     0x0
gen_isr_tables.py: 0x7e21     11  0     0x0
gen_isr_tables.py: 0x7e5f     1   1     0x0

Signed-off-by: Sebastian Bøe <edee888afd63e94916688495334700803eba20a0@nordicsemi.no>

Vudentz/zephyr,Vudentz/zephyr,nashif/zephyr,explora26/zephyr,galak/zephyr,ldts/zephyr,zephyriot/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,mbolivar/zephyr,punitvara/zephyr,galak/zephyr,explora26/zephyr,explora26/zephyr,ldts/zephyr,galak/zephyr,mbolivar/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,nashif/zephyr,explora26/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,mbolivar/zephyr,kraj/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,Vudentz/zephyr,punitvara/zephyr,mbolivar/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,ldts/zephyr,zephyriot/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyriot/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,Vudentz/zephyr,mbolivar/zephyr,kraj/zephyr,kraj/zephyr,zephyriot/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,kraj/zephyr,explora26/zephyr,finikorg/zephyr,finikorg/zephyr,nashif/zephyr