We should avoid using x16~x31 register in program buffer because there
are no such general purpose registers in RVE(Embedded) extension.
For targets that support rvE, when the parameter increment=0
and count>1 of the read_memory_progbuf function, openocd will cause
an error due to the use of the s2 register.
For example:
{Command} {riscv repeat_read} count address [size=4]
Change-Id: I8b74dcc15cd00a400f2f1354c577a82132394435
Signed-off-by: Hang Xu <xuhang@eswincomputing.com>