lib: sbi: Detect number of bits implemented in mhpmcounter

RISC-V privilege specification allows the implementation to have less
than 64 bits.

Add a function to detect the number of implemented bits in mhpmcounter
dynamically at runtime.

Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>
Signed-off-by: Atish Patra <atish.patra@wdc.com>