board: phytec: phycore-imx93: Add VOLT_LOW_DRIVE frequency fixup

board: phytec: phycore-imx93: Add VOLT_LOW_DRIVE frequency fixup

For phyCORE-i.MX93 SoMs with i.MX93 parts running in VOLT_LOW_DRIVE mode
(SoCs with speed grade fuse set to 900 MHz) reduce usdhc clocks from 400
MHz to 266 MHz. Do this in board code since global imx9 board_fix_fdt()
is not used in case of phycore-imx93 board since commit d3b9b7996889
("board: phytec: imx93: Add eeprom-based hardware introspection").

While at it, add a note to ft_board_setup() function to inform that
fixup for Linux device-tree is taken care by ft_system_setup() in imx9
global arch/arm/mach-imx/imx9/soc.c implementation.

Signed-off-by: Primoz Fiser <primoz.fiser@norik.com>