lmb: riscv: Add arch_lmb_reserve()

lmb: riscv: Add arch_lmb_reserve()

Add arch_lmb_reserve() implemented using arch_lmb_reserve_generic().
It is rather likely this architecture also needs to cover U-Boot with
LMB before booting Linux.

Reviewed-by: Rick Chen <rick@andestech.com>
Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Atish Patra <atish.patra@wdc.com>
Cc: Leo <ycliang@andestech.com>
Cc: Rick Chen <rick@andestech.com>
Cc: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Cc: Tom Rini <trini@konsulko.com>