arm64: mm: drop unused __pa(__idmap_text_start)

x5 is not used in the following map_memory. Instead,
__pa(__idmap_text_start) is stored in x3 which is used later.

Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20210518101405.1048860-4-aisheng.dong@nxp.com
Signed-off-by: Will Deacon <will@kernel.org>