net: stmmac: move stmmac_bus_clks_config() to stmmac_platform.c

stmmac_bus_clks_config() is only used by stmmac_platform.c, so rather
than having it in stmmac_main.c and needing to export the symbol,
move it to where it's used.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/E1v119j-0000000773s-1R2v@rmk-PC.armlinux.org.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>