This pass has some more support for using the Hardware Serial objects.

Including setting an ISR for doing both reads and writes,  sets up the FIFO,

Sitll lots more work to go.

Things like: fifo - need to better understand/set watermarks, also RX need to read in more than one byte if more available.

Also support fo r other features, like formats, 9 bit, ...

BUt at least you can do simple reads/writes

shifting both sides by 16 bit gives pretty good accuracy with fast code:
        movw    r3, #9155
        ldr     r2, .L2
        mul     r0, r3, r0
        lsrs    r0, r0, #1
        umull   r3, r0, r2, r0
        lsrs    r0, r0, #8
.L2:
        .word   144122641

with 984 MHz it allows nsec to be max ~143000  max which is plenty

This makes sense now, and would be useful for shorter wait-times. Could be used for PulseIn etc.  where we need defined very short delays.

- comment SRC_GPR1..SRC_GPR10 (used by the ROM code)

Deleted a duplicate define: #define CCM_CBCMR_PERIPH_CLK2_SEL_MASK		((uint32_t)(0x03 << 12))
 and added tempmon registers