ARM: imx8mm: verdin-imx8mm: Rework board_early_init()

ARM: imx8mm: verdin-imx8mm: Rework board_early_init()

Rename board_early_init_f() to board_early_init(), since this function
has nothing to do with actual board_early_init_f() as used throughout
U-Boot. The board_early_init() is function local to this board used to
configure UART and WDT pinmux. Wrap init_uart_clk() into this function
so that early UART init would be all in one place. Turn the function
into __weak one, so it could be overridden in case custom carrier board
uses different UART or needs custom IOMUX settings.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <festevam@denx.de>
Cc: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Cc: Max Krummenacher <max.krummenacher@toradex.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>