spi: omap2-mcspi: Fix hardcoded reference clock

spi: omap2-mcspi: Fix hardcoded reference clock

A hardcoded reference clock of 48 MHz is used to calculate the
clock divisor values, but the reference clock frequency can be
different across devices and can be configured which can cause
a mismatch between the reported frequency and actual SPI clock
frequency observed. Fix this by fetching the clock rate from
the clock provider and falling back to hardcoded reference only
if the clock is not supplied.

Fixes: 2cd7d393f461 ("arm64: dts: ti: k3-am654: Add McSPI DT nodes")
Signed-off-by: Vaishnav Achath <vaishnav.a@ti.com>
Reviewed-by: Dhruva Gole <d-gole@ti.com>
Link: https://lore.kernel.org/r/20230926113812.30692-1-vaishnav.a@ti.com
Signed-off-by: Mark Brown <broonie@kernel.org>