lib: sbi: check incoming dbtr shmem address

lib: sbi: check incoming dbtr shmem address

Current Debug Trigger SBI extension proposal suggests to activate
shmem area and obtain its physical address from S-mode software
in the following way:

: If both `shmem_phys_lo` and `shmem_phys_hi` parameters are not
: all-ones bitwise then `shmem_phys_lo` specifies the lower XLEN
: bits and `shmem_phys_hi` specifies the upper XLEN bits of the
: shared memory physical base address. The `shmem_phys_lo` MUST
: be `(XLEN / 8)` byte aligned and the size of shared memory is
: assumed to be `trig_max * (XLEN / 2)` bytes.

For more details see the current version of the proposal:
- https://lists.riscv.org/g/tech-debug/message/1302

On the other hand, on RV32, the M-mode can only access the first 4GB of
the physical address space because M-mode does not have MMU to access
full 34-bit physical address space. Similarly, on RV64, the M-mode can
only access memory addressed by 64 bits.

This commit checks shmem address in function sbi_dbtr_setup_shmem
to make sure that shmem_phys_hi part of the valid address is zero.
Besides, the macro DBTR_SHMEM_MAKE_PHYS is updated to take into
account only low XLEN part.

Signed-off-by: Sergey Matyukevich <geomatsi@gmail.com>
Reviewed-by: Himanshu Chauhan <hchauhan@ventanamicro.com>