Add conversion macros from milliseconds to various units.
The purpose of these macros is to make it more clear/easier
for users to set and read values using milliseconds rather
than the various BT units which may be in 0.625, 1.25 or 10ms
units.
This is especially useful when comparing related values using
different units, such as advertising interval (0.625ms units)
and periodic advertising interval units (1.25ms units).
Users will have to be aware that these macros can provide slightly
different values than what is provided, if the provided values
do not match the units.
Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>