phy: phy-rockchip-samsung-hdptx: Select CONFIG_MFD_SYSCON

Compile testing configurations without REGMAP support enabled results in
a bunch of errors being reported:

  ../drivers/phy/rockchip/phy-rockchip-samsung-hdptx.c:569:21: error: variable ‘rk_hdptx_phy_regmap_config’ has initializer but incomplete type
    569 | static const struct regmap_config rk_hdptx_phy_regmap_config = {
        |                     ^~~~~~~~~~~~~
  ../drivers/phy/rockchip/phy-rockchip-samsung-hdptx.c:570:10: error: ‘const struct regmap_config’ has no member named ‘reg_bits’
    570 |         .reg_bits = 32,
        |          ^~~~~~~~

Note that selecting REGMAP alone is not enough, because of the following
liker error:

  phy-rockchip-samsung-hdptx.c:(.text+0x10c): undefined reference to `__devm_regmap_init_mmio_clk'

Instead of the obvious fix to enable REGMAP_MMIO, select MFD_SYSCON,
which implicitly enables REGMAP_MMIO as well.  The rationale is that the
driver has been already relying on the syscon functionality.

Moreover, without MFD_SYSCON enabled, the test coverage is reduced,
since the linker might not detect any potential undefined references
following syscon_regmap_lookup_by_phandle() invocation in
rk_hdptx_phy_probe() body.  That is because the function would
unconditionally return -ENOTSUP, hence the compiler is free to optimize
out any unreachable code.

Finally ensure PHY_ROCKCHIP_SAMSUNG_HDPTX depends on HAS_IOMEM, as
required by MFD_SYSCON.

Fixes: 553be283

 ("phy: rockchip: Add Samsung HDMI/eDP Combo PHY driver")
Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Link: https://lore.kernel.org/r/20240629-rk-hdptx-compile-test-fix-v1-1-c86675ba8070@collabora.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>