soc: sunxi: sram: Prevent the driver from being unbound

This driver exports a regmap tied to the platform device (as opposed to
a syscon, which exports a regmap tied to the OF node). Because of this,
the driver can never be unbound, as that would destroy the regmap. Use
builtin_platform_driver_probe() to enforce this limitation.

Fixes: 5828729b ("soc: sunxi: export a regmap for EMAC clock reg on A64")
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Link: https://lore.kernel.org/r/20220815041248.53268-5-samuel@sholland.org