libbladeRF: add si5338 peek functionality
bladeRF-cli: add MIMO clock settings

This commit introduces the first of many MIMO functionalities. MIMO clock
settings will have to be moved to libbladeRF eventually.
Setting the device as a MIMO slave makes the Si5338 use the SMB port a
reference. This is done by routing single-ended IN4 to PFD_IN_REF by setting
registers 28 to 30 to the following values:

PFD_IN_REF[2:0] = 001 (P2DIV_IN)
P2DIV_IN[2:0] = 011 (Clock from IN4 is input to P2)

reg(28) 0x2b  = | 0 | 0 | 1 | 0   1   0 | 1   1 |
P2DIV_IN[0] = 1
P1DIV_IN[2:0] = 010

reg(29) 0x28  = | 0   0   1 | 0   1 | 0   0   0 |
PDF_IN_REF[2:0] = 001
P1DIV_IN[4:3] = 01
P1DIV[2:0] = 000

reg(30) 0xa8  = | 1   0   1 | 0   1 | 0   0   0 |
PDF_IN_FB[2:0] = 101
P2DIV_IN[2:1] = 01
P2DIV[2:0] = 000

Register 6 is set to 4 so that the Si5338 loss of signal indicator knows the PFD
is using IN4 (FBCLK) as a reference.

Setting the device as a MIMO master makes the Si5338 drive the SMB port from
CLK3A.