serial: stm32: get FIFO size from hwcfg register

Since STM32MP25, FIFO size could vary regarding the STM32MPxx version.
So we get this size from "hwcfgr1" register and compute threshold values
corresponding to the ratio given by reference manual.

As STM32MP1x, STM32MP25 and STM32H7 share the same compatible and STM32H7
doesn't have a register to get FIFO size, we force FIFO size to 16 in case
of zero read from hwcfgr1 register.

Signed-off-by: Valentin Caron <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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