MFC r354554-r354558, r355115

r354554 by mmel:
Implement support for (soft)linked clocks.
This kind of clock nodes represent temporary placeholder for clocks
defined later in boot process. Also, these are necessary to break
circular dependencies occasionally occurring in complex clock graphs.

r354555 by mmel:
Cleanup Rockchip clocks implementation.

• style
• unify dprinf defines
• make dprinf's 32-bit compatible

Not a functional change.

Reviewed by: manu, imp
Differential Revision: https://reviews.freebsd.org/D22281

r354556 by mmel:
Enhance Rockchip clocks implementation.

• add support for fractional dividers
• allow to declare fixed and linked clock

Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D22282

r354557 by mmel:
Tidy up Rockchip composite clock.

• add support for log2 based dividers
• use proper write mask when writing to divider register

Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D22283

r354558 by mmel:
Remove explicit declaration of rk_clk_fract_set_freq() function
forgotten in r354556.

MFC with: r354556
Noticed by: manu

r355115 by mmel:
Finish implementation of RK3299 clocks.

• implement of all but mmc clocks. MMC clocks will be added later by own commit.
• use 'link' clock type for external clocks.
• use macros for initialization of structure's named members.

Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D22441