configs: ibex-ast2700: Explicitly disable Zaamo and Zalrsc extension

configs: ibex-ast2700: Explicitly disable Zaamo and Zalrsc extension

This board supports neither Zaamo nor Zalrsc extension, thus we want to
build it without "a" specified in the ISA string passed to compiler in
case of misused A-extension instructions. With RISCV_ISA_ZAAMO and
RISCV_ISA_ZALRSC Kconfig options introduced, we must explicitly disable
both of them to achieve this.

Signed-off-by: Yao Zi <ziyao@disroot.org>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>