tests: power_mgmt_soc: Increase idle stack size

Test suite test_power_mgmt succeeded
E: ***** USAGE FAULT *****
E:   Illegal use of the EPSR
E: r0/a1:  0x00000000  r1/a2:  0x00118598  r2/a3:  0x00118598
E: r3/a4:  0x00000000 r12/ip:  0x00000000 r14/lr:  0x000e246f
E:  xpsr:  0x60000000
E: Faulting instruction address (r15/pc): 0x00000014
E: >>> ZEPHYR FATAL ERROR 0: CPU exception on CPU 0
E: Current thread: 0x00118598 (main)
E: Halting system

Fixes: #30192

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>