additional SPI clock choices in SPI.cpp

[manitou48]

In forum discussions, a user noted that with T3.6@180mhz (bus 60 mhz), requesting a SPI clock of 4 MHz did not give exactly 4 mhz. Originally, i thought it was just discretization error based on available dividers. But i have a perl script that will give me the "best" CTAR settings for given bus speed and desired SPI clock. There is a CTAR setting (0xb80020002) that gives 4mhz, but it's not in the ctar_div_table in SPI.cpp. You might wish to expand the div and clock table. You have 23 entries, but there are 42 entries with BR in the range 0 to 8.

2 3 4 5 6 7 8 9 10 12 14 15 16 18 20 21 24 28 30 32 40 42 48 56 64 80
96 112 128 160 192 224 256 320 384 448 512 640 768 896 1280 1792

https://forum.pjrc.com/threads/40394-SPI-clock-not-uniform