drm: renesas: shmobile: Rename input clocks

drm: renesas: shmobile: Rename input clocks

Prepare for DT bindings by using more appropriate names for the input
clocks.

Note that all LDDCKR_ICKSEL_* definitions but the one for the bus clock
are valid only for SH7724, so the clock selection code needs to be
updated when extending clock support to other SoCs.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
[geert: Add note]
Reviewed-by: Sui Jingfeng <suijingfeng@loongson.cn>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/d4b4d2164541ae055d10064103db3c2d6540e846.1694767209.git.geert+renesas@glider.be