CONFIG_U5500_MODEM_IRQ is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Modem IRQ support

modulename: modem-irq-db5500.ko

configname: CONFIG_U5500_MODEM_IRQ

Linux Kernel Configuration
└─>Modem IRQ support
In linux kernel since version 2.6.37 (release Date: 2011-01-04)  
Add support for handling IRQ:s from modem side

source code:
arch/arm/mach-ux500/modem-irq-db5500.c