The Broadcom Bluetooth controllers can take up to two external clocks:
an external frequency reference, substituting the main crystal, and a
LPO clock at 32.768 kHz substituting the internal LPO clock.

In particular, the external LPO clock must be used when the controller
does not have NVRAM connected, and the main reference frequency is not
the default 20 MHz. This is described in detail in the datasheet.

The original "extclk" clock name is ambiguous as to which of these it
refers to, and some designs might even require both.

This patch deprecates the existing name, and adds "txco" and "lpo".
Tested-by: Ondrej Jirman <megous@megous.com>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>