Add contants and update comment about ALP

This commit add two constants and updates the comment about ALP.

It was not easy to find the definition of ALP but after searching I
found what I believe is the correct definition in section 3.3 "Clocks"
in the referenced document below.

Active Low Power (ALP):
Supplied by an internal or external oscillator. This clock is
requested by cores when accessing backplane registers in other cores
or when performing minor computations. When an external crystal is
used to provide reference clock, ALP clock frequency is determined by
the frequency of the external oscillator. A 37.4 MHz reference clock
 is recommended.

Refs:
https://www.infineon.com/dgdl/Infineon-AN214828_Power_Consumption_Measurements-ApplicationNotes-v03_00-EN.pdf?fileId=8ac78c8c7cdc391c017d0d2803a4630d

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