MIPS: lantiq: Shorten register names, remove unused macros

The macros LTQ_ICU_IM1_ISR and LTQ_ICU_OFFSET seems to be unused, remove
them. Allong with that, remove _IM0 substring from the macro names. The
IM (interrupt module) is already defined in IOMEM access and IM0 would be
misleading.

Signed-off-by: Petr Cvek <petrcvekcz@gmail.com>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: hauke@hauke-m.de
Cc: john@phrozen.org
Cc: linux-mips@vger.kernel.org
Cc: openwrt-devel@lists.openwrt.org
Cc: pakahmar@hotmail.com