OMAP2 clock: 2430 I2CHS uses non-standard CM_IDLEST register

OMAP2430 I2CHS CM_IDLEST bits are in CM_IDLEST1_CORE, but the CM_*CLKEN bits
are in CM_{I,F}CLKEN2_CORE [1].  Fix by implementing a custom clkops
.find_idlest function to return the correct slave IDLEST register.

...

1. OMAP2430 Multimedia Device Package-on-Package (POP) Silicon Revision 2.1
   (Rev. V) Technical Reference Manual, tables 4-99 and 4-105.

Signed-off-by: Paul Walmsley <paul@pwsan.com>