Add LMH6518 datasheet to repo and SPI core/simple test over JTAGBone.

[enjoy-digital]

LMH6218 is a Programmable Gain Amplifier, present on each of the 4 channels. IN_P is connected to the Frontend. IN_P is connected to TRIM (https://github.com/enjoy-digital/thunderscope/issues/5). The configuration is done over SPI with common SDIO/SCLK for all chips and separate CSn. Let's first do a simple unit-test over JTAGBone and verify outputs with a scope.

[enjoy-digital]

LMH6518 configuration from official firmware:

// PGA LMH6518
enum ThunderScopeHWStatus thunderscopehw_set_pga(struct ThunderScopeHW* ts, int channel)
{
    uint8_t fifo[4];
    fifo[0] = 0xFB - channel;  // SPI chip enable
    fifo[1] = 0;
    fifo[2] = 0x04;  // ??

    int vdiv = ts->channels[channel].vdiv;
    if (vdiv > 100) {
        // Attenuator relay on, handled by
        // thunderscopehw_set_datamover_reg.
        vdiv /= 100;
    }

    switch (vdiv) {
    case 100: fifo[3] = 0x0A; break;
    case  50: fifo[3] = 0x07; break;
    case  20: fifo[3] = 0x03; break;
    case  10: fifo[3] = 0x1A; break;
    case   5: fifo[3] = 0x17; break;
    case   2: fifo[3] = 0x13; break;
    case   1: fifo[3] = 0x10; break;
    default: return THUNDERSCOPEHW_STATUS_INVALID_VDIV;
    }
    switch (ts->channels[channel].bw) {
    case  20: fifo[3] |= 0x40; break;
    case 100: fifo[3] |= 0x80; break;
    case 200: fifo[3] |= 0xC0; break;
    case 350: /* 0 */ break;
    default: return THUNDERSCOPEHW_STATUS_INVALID_BANDWIDTH;
    }
    return thunderscopehw_fifo_write(ts, fifo, 4);
}

[enjoy-digital]

LMH6518 added totest_frontend with https://github.com/enjoy-digital/thunderscope/commit/e6f17f3e9b3b61dfd3d5bcf4b1612af5dbb11993. SPI/Configuration seems to be done correctly when looking at PGA output with a scope.

[enjoy-digital]

Done.