net: stmmac: add documentation for clocks

net: stmmac: add documentation for clocks

Add documentation covering stmmac_clk, pclk, clk_ptp_ref and clk_tx_i
in the hope that this will help understand what each of these clocks
are for.

There is confusion around stmmac_clk and pclk which can't be easily
resolved today as the Imagination Technologies Pistachio board that
pclk was introduced for has no public documentation and is likely now
obsolete. So the origins of pclk are lost to the winds of time.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Tested-by: Mohd Ayaan Anwar <mohd.anwar@oss.qualcomm.com>
Link: https://patch.msgid.link/E1vzX5Z-0000000CVsb-1XTm@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>