arm: Fix spi_in_cell for SPIs 32..63

arm: Fix spi_in_cell for SPIs 32..63

The cell configuration format restricts us to 64 SPIs this far. Make
sure that we properly test the range of 32 to 63 and avoid overflows due
to 32-bit word width. As Jailhouse provides no __aeabi_llsr, extract
high and low words first, then scan within 32 bits.

This addresses Coverity finding CID 21110.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>