pm: handle the case of 2MHz frequency
pm: handle the case of 2MHz frequency

It is not possible to generate 1 MHz REF_TICK from 2 MHz APB clock
(this is a limitation of REF_TICK divider circuit). Since switching
REF_TICK frequency is something we would like to avoid (to maintain
UART output even with DFS), 2 MHz frequency has been marked as
unsupported.

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