power: pmic: rk8xx: fix duplicate prompt

SPL_PMIC_RK8XX and PMIC_RK8XX both share the same prompt making it
difficult to know at first glance in menuconfig what's for what, let's
fix this by adding "in SPL" at the end of the prompt for the SPL symbol.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>