gpio: thunderx: remove unused .map() hook from irq_domain_ops

This driver implements .alloc() hook, so .map() is not used.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: David Daney <david.daney@cavium.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>