ANDROID: regmap: Unhide REGMAP_SPMI

Nothing in GKI currently selects/depends on this option, but the ship
has sailed on building it in via Kconfig.gki, so just unhide the symbol.

This enables downstream targets to select/depend on it with a defconfig
fragment without needing to patch the core kernel.

Bug: 248307451
Signed-off-by: Alistair Delva <adelva@google.com>
Change-Id: Iff9f56027ce5eeb42360a018bda755970331c984