sysreset: syscon: remove POWER reset cause

The sysreset of 'POWER' type is a PMIC reset defined as
'remove and restore power'.
So normally only COLD/WARN is supported by sysreset_syscon.

This modification allows to use the function sysreset_walk(SYSRESET_POWER)
when it is supported by PMIC driver (see example in stpmic1).

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>