clk: stm32mp1: add support of missing SPI clocks

clk: stm32mp1: add support of missing SPI clocks

Add the missing SPI clock even if these instances are not available
on STMicroelectronics boards: SPI2_K, SPI3_K, SPI4_K, SPI6_K.

With this patch, the SPI2 / SPI3 / SPI4 / SPI6 instances can be used on
customer design without the clock driver error:
  stm32mp1_clk_get_id: clk id 131 not found

Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>