ux500: rename modem IRQ and MBOX files

Suffix the U5500 modem IRQ and MBOX files with *-db5500* so that
we clearly know the SoC they belong to, in line with the rest of
the files in mach-ux500.

Cc: Stefan Nilsson <stefan.xk.nilsson@stericsson.com>
Cc: Martin Persson <martin.persson@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>