drm/stm: dsi: expose DSI PHY internal clock

DSISRC __________
	               __\_
	              |    \
	pll4_p_ck   ->|  1  |____dsi_k
	ck_dsi_phy  ->|  0  |
	              |____/

A DSI clock is missing in the clock framework. Looking at the
clk_summary, it appears that 'ck_dsi_phy' is not implemented. Since the
DSI kernel clock is based on the internal DSI pll. The common clock
driver can not directly expose this 'ck_dsi_phy' clock because it does
not contain any common registers with the DSI. Thus it needs to be done
directly within the DSI phy driver.

Signed-off-by: Raphael Gallais-Pou <raphael.gallais-pou@foss.st.com>
Acked-by: Yannick Fertre <yannick.fertre@foss.st.com>
Tested-by: Yannick Fertre <yannick.fertre@foss.st.com>
Signed-off-by: Philippe Cornu <philippe.cornu@foss.st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240129104106.43141-4-raphael.gallais-pou@foss.st.com